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Quality Open Source Calendaring / Scheduling?

Posted by kdawson on Thu Nov 29, 2007 12:30 PM
from the date-for-a-date dept.
Jim R. Wilson writes "In past jobs, I've used Microsoft Outlook/Exchange, Novell Groupwise, and Google Calendar for handling business appointments. I'm sorry to say it, but I have yet to see a rival to Microsoft's scheduling features. On Slashdot I have occasionally read rumblings that there are better open source email and calendaring solutions out there. Can anyone substantiate this claim? What are the OSS alternatives? Can any compete with Microsoft's resource scheduling?"
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  • no (Score:5, Informative)

    by Anonymous Coward on Thursday November 29 2007, @12:32PM (#21520305)
    no
    • Re:no (Score:5, Insightful)

      by Crispin Cowan (20238) <crispinNO@SPAMcrispincowan.com> on Thursday November 29 2007, @01:50PM (#21521715) Homepage
      Who ever moderated the parent as "troll" didn't think about it very hard. A simple "no" in answer to this question is actually quite accurate. That is sad, and there is a great deal more to be said on the matter, but it is the truth.
      • Re:Zimbra! (Score:4, Funny)

        by michrech (468134) on Thursday November 29 2007, @04:06PM (#21523795)
        No.

        Zimbra does it, does it very well and can be made compatible with full fledged outlook.

        You dweebs cant even do a google search before just saying "no", can you?
  • Haven't found much (Score:4, Informative)

    by cbreaker (561297) on Thursday November 29 2007, @12:32PM (#21520309) Journal
    I haven't found much, either. It's either some half-done web-based solution or it's got seriously missing features.

    Evolution works great with Exchange; all they need now is to create their own back-end =)

    PS. Public folders have gone away in Exchange 2007; big mistake if you ask me. It was a selling point for Exchange.
    • by Anonymous Coward on Thursday November 29 2007, @12:51PM (#21520683)
      Sure about public folders?

      http://www.microsoft.com/exchange/evaluation/topquestions.mspx?wt.svl=overview [microsoft.com]

      Q. What is happening with Public Folders?
      A.

      Public Folders are included and supported in Exchange Server 2007. Microsoft has communicated that future releases of Exchange Server may not include public folders. If you use Public Folders, read the Exchange Team Blog on the topic of public folders for more guidance.
    • Re: (Score:3, Funny)

      I wrote a calendaring web app a few years back, and it was certainly a half-done web-based solution some seriously missing features. I wish you had included mine in your survey, because I still don't have any customers for it.
    • by ibi (61235) on Thursday November 29 2007, @12:58PM (#21520823)
      It's worth looking at Chandler (fat client) and Cosmo (server) from

      http://www.osafoundation.org/ [osafoundation.org]

      It's been a long time coming, but it's finally approaching a useable release and it's quite interesting. I think it will be a real choice in 2008...
    • by samkass (174571) on Thursday November 29 2007, @01:32PM (#21521377) Homepage Journal
      The best candidate I've seen for the full calendaring "infrastructure" is the open CalDAV [wikipedia.org] spec, but it's only really used commercially by Apple at this point. But since Apple has released their reference implementation as open source [wikipedia.org], perhaps we'll get more implementations and a snowball effect of support.
          • by Crispin Cowan (20238) <crispinNO@SPAMcrispincowan.com> on Thursday November 29 2007, @02:04PM (#21521957) Homepage

            That is so painfully wrong.

            Paper calendars work great for scheduling with the rest of your family, because you all pass through the kitchen. But that does not scale to large enterprises, you know, like with more than 50 people. It does not scale to distributed organizations, where you don't share a kitchen. It does not connect appointment scheduling to nag 'bots that remind you to attend the meeting.

            But I think this is the core reason why open source calendaring sucks: it is a problem that most open source community people don't have, and only really is a problem in large organizations.

            Sadly, this has lead to open source completely failing to take over the mail server market. Linux & *BSD, Postfix, and Qmail all make great mail servers, and are used by many ISPs, but they are largely unused in enterprises, precisely because of the lack of calendaring. As a result, corporate mail servers are invariable Microsoft Exchange, Lotus Domino, or Novell Groupwise.

            Hula [wikipedia.org] was an attempt to address this, but either due to Novell not doing it right or the community just not caring, it did not work out so well [novell.com] :-(

            I would really like to see the open source community get this right. If we don't, then the mail server market will continue to be dominated by proprietary products.

            • Actually, yes... perhaps. For whatever reason, Novell decided to let go of the Hula project but, being open source, others took the project on and it's alive (although it seems to be progressing forward slowly due to lack of man power). It's called Bongo [bongo-project.org], and it looks pretty nice. Go check it out.
            • by greginnj (891863) on Thursday November 29 2007, @04:17PM (#21523923) Homepage Journal
              Pencils are dangerous in space.

              1. They are more pointed than pens, and thus more likely to puncture things that shouldn't be punctured.

              2. They create dust, which is a no-no on space missions. Wood pencils (obviously) from sharpening. Mechanical pencils are prone to have their leads break off, and float about. More to the point, the operating mechanism of both kinds of pencil is to rub off graphite dust onto paper. Some of this dust may be released by smudging.

              Remember that graphite, and thus graphite dust, is conductive. Do you want to take the risk of conductive graphite dust causing a component to short out?

              Why do you want NASA missions to fail???? ( oblig bit o funny )

  • by Anonymous Coward on Thursday November 29 2007, @12:33PM (#21520321)

    Application: Pen and Paper.

    • by PrescriptionWarning (932687) on Thursday November 29 2007, @12:55PM (#21520753)
      Unfortunately its not chicken scratch resistant
    • by Eberlin (570874) on Thursday November 29 2007, @01:06PM (#21520957) Homepage
      As much as I love the Y2K compliant Office App...

      0) Geeks will argue which pen and which paper is the best.
      1) The Gentoo crowd will make their own paper from pulp.
      2) Where's the ^H on Pen?
      3) There are some serious latency issues
      4) Sometimes the output is so horrible that others can't read the file.
      5) Sometimes the output is so horrible that I can't read my own file.
      6) You can backspace on a word processor. You can shake an etch-a-sketch. If you mess up on paper, you need new hardware.
        • by tuomoks (246421) <tuomo@descolada.com> on Thursday November 29 2007, @07:43PM (#21526681) Homepage
          Rated as funny and, yes, it seems funny today. I'm from time we had secretaries and I miss those times. Scheduling between internal and external (customer) connections is PITA. The secretaries were able to negotiate, no system can do that, it is out of your time. They knew personal things what you never would put to any scheduling system, bring flowers or a bottle of whiskey to next meeting because the big boss has a birthday, whatever. They reminded you of all the papers you will need to take to the meeting, they had your flights ready, they proposed to have another person to go with you or just sending another person because whatever reason, etc. No calendar, scheduling, computer system can never do that and it is out of your time - sometimes a lot! So hire an assistant!
  • Compatibility (Score:5, Insightful)

    by WPIDalamar (122110) on Thursday November 29 2007, @12:34PM (#21520337) Homepage
    I think the main problem is we can't really come up with an open source scheduling system that's compelely new and innovative because you need compatibility with people outside your organization.

    If we're not coming up with something new and innovative we're stuck making outlook clones. People don't like writing software like that.
      • Re:Compatibility (Score:5, Interesting)

        by forrestt (267374) on Thursday November 29 2007, @01:06PM (#21520947) Homepage Journal
        How is that an assumption about your scheduling? It's an invitation to a meeting, if you can't make it or don't want to, you are free to decline the invitation or even just ignore it. But, it would be nice if your dentist could send you an appointment reminder with a link that would put it in your calendar so when your boss is wondering where you are he can look at the calendar (no, telling your boss has no effect on them knowing where you are when they want you). Or perhaps your friend could send you an email to go do something that would require you to take off early Friday and include a link to update your calendar. Or maybe some vendor could send you an invitation to meet them for lunch with multiple times for the event and you could pick one. Or maybe a customer needs to meet you to schedule a time they can call you so you send them a meeting invite, Or maybe even the people from SANS sending you an email after you register with a link to update your calendar to say you won't be at work for that week.

        Being in your company has nothing to do with wanting information in your calendar, and you are the person that gets to decide if it is worth putting in the calendar or not.
        • Re: (Score:3, Insightful)

          by brunascle (994197)
          you can just put them in the calendar yourself. that's what i do.

          if my dentist sent me a meeting invite, it will not be how i want it. he'll likely mark it as "busy" or "out of office" for the exact time of the meeting, with the default 15 mins reminder. that's not how i want it. i'll need it to expand much further than that, because it takes at least an hour to get from work to there; and i'll want a 1 week reminder.

          i agree with the GP. sender-created meeting invites work fine for the office, but not
      • Re: (Score:3, Funny)

        by toleraen (831634)

        You're not in my company; don't make assumptions about my scheduling.
        Yeesh, I take it you don't get too many party invitations in the mail? Your Friday nights are booked up all the way through 2009? How dare someone try to include you in a gathering that might be relevant to your interests!

        That's why, at least in Outlook, there are "Propose New Time" and "Decline" buttons. We should get together next Tuesday at 1330 so I can show you these features.
  • by guysmilee (720583) on Thursday November 29 2007, @12:35PM (#21520367)
    Citadel is the best i know of: http://www.citadel.org/doku.php [citadel.org]
  • What features? (Score:5, Insightful)

    by Spy der Mann (805235) <spydermann.slashdot@NoSpAm.gmail.com> on Thursday November 29 2007, @12:35PM (#21520369) Homepage Journal
    It's hard to expect the developers to write a feature they haven't been asked about, and/or don't even know it exists.

    In other words, what features do you use in MS products that you haven't found in the free/open source applications?
    • Re:What features? (Score:5, Informative)

      by jimbojw (1010949) <wilson.jim.rNO@SPAMgmail.com> on Thursday November 29 2007, @12:58PM (#21520817) Homepage

      In other words, what features do you use in MS products that you haven't found in the free/open source applications?

      Sorry I wasn't clear enough in my initial question. What really impresses me about Outlook/Exchange is when you go to schedule a meeting, it allows you to see when all the participants, rooms and resources (like projectors) are available in a horizontal chart of sorts. People who are busy are marked off in blue, out of office is purple, etc. To find a time that works for everyone, you just scan across until you see a vertical bar of white (everyone free), or try to minimize conflicts.

      I don't know of MS holds a patent on the UI, but I haven't seen it anywhere else. Also, with respect to calendaring, in Outlook you can open up several calendars (yours and others) side-by-side in order to see who's free when. It's a pretty simple bit of eye candy, but nonetheless, I've only seen it in Outlook.

      • Re:What features? (Score:5, Informative)

        by 99BottlesOfBeerInMyF (813746) on Thursday November 29 2007, @01:27PM (#21521311)

        I don't know of MS holds a patent on the UI, but I haven't seen it anywhere else.

        This function is available with the CalDav server standard and that particular feature is available in the implementations in the open source Zimbra client/server and the 10.5 version Apple's iCal server/client. I don't know about other implementations, but I imagine most other ones either include this or will soon, as Caldav has really taken of in adoption by major projects. Zimbra even offers that feature via the Web interface to their server.

        Also, with respect to calendaring, in Outlook you can open up several calendars (yours and others) side-by-side in order to see who's free when.

        I think Evolution has an interface like this (works with CalDav), but if I recall Zimbra allows you to overlap as many calendars as you want in one window, making the comparison quite a bit easier IMHO.

        I'd definitely look at Zimbra if you're serious about a OSS solution with lots of features and compatibility with both standards and proprietary interfaces (they have a full featured Exchange plug-in so users can still use Exchange as their client if they want). The server will run on all the popular Linux distros, OS X, and as a VMWare appliance.

  • WebCalendar (Score:3, Interesting)

    by DaGoodBoy (8080) on Thursday November 29 2007, @12:35PM (#21520377) Homepage
    We use this: http://www.k5n.us/webcalendar.php [k5n.us]

    Works well for our needs.
  • by narrowhouse (1949) on Thursday November 29 2007, @12:37PM (#21520397) Homepage
    Jim,

    I hate to say this, but unless you give us a few reasons why some of the solutions you have looked at are not sufficient I doubt you will get any meaningful response.It's a pretty common problem when people ask for an open source replacement for a program they have used and were reasonably happy with.

    Without some starting point for comparison you will just get dozens of stories about how product X works fine for them.
    • by whoever57 (658626) on Thursday November 29 2007, @01:47PM (#21521665) Journal
      How about this:
      1. It needs to have a client/server architecture (for mobile clients who don't have always-on connectivity). Pure web-based calendars don't do this.

      2. It needs to have Windows and Linux clients.

      3. Outlook plug-ins don't work. This is a limitation of Outlook. The plug-in can't be the default calendar, and Outlook will only pop up reminders for the default calendar. Also, my experience of OpenGroupware's plug-in is that it is unstable.

      4. It needs to have a means for one person to schedule an event on someone-else's calendar (if the appropriate permissions are given).

      5. It needs to have a way for people to view the details of other people's calendars (if the appropriate permissions are given). Free/Busy information is not enough in some cases.

      If someone can tell me of a calendar system that meets these requirements, I would be thrilled!

      Oh, one more -- it desn't need to try to replace other things, such as email servers, etc..

  • This is Slashdot. (Score:4, Interesting)

    by jellomizer (103300) * on Thursday November 29 2007, @12:37PM (#21520407)
    Just because it is by microsoft people hate the product even if they never used it before. They will say Some Obscure Open Source tool is better even though they never really used the microsoft one... After so they just may realize that they are missing someting. That is the last thing they want to hear. It would be like someone from an other political party saying someone from the other party actually made a big difference and the world is better because of him/her. It just wont happen.
    • Re: (Score:3, Insightful)

      by orclevegam (940336)

      I've used Outlook before. I used it for over four years as it's the official corporate e-mail/scheduling client. The scheduling they did a pretty good job on, I'll give them that, but as an e-mail client I've never cared for it. I much prefer thunderbird or the web interface on gmail. Really the question people are looking for is, how do we replace the scheduling portion of Outlook and still retain all it's nice features while using the e-mail client of our choice?

      This is particularly tricky because one of

    • Re: (Score:3, Interesting)

      by srussell (39342)

      Just because it is by microsoft people hate the product even if they never used it before.

      I have to use Exchange at work every day. It sucks. My main gripes are:

      • It doesn't integrate very well with anything but Outlook
      • It often has problems with timezones and/or time changes. We get a week or two of screwed up scheduling twice a year, right around the daylight savings time change.
      • Resource scheduling is just stupid. If you forget and add a room as a required participant instead of a resource, it doesn
  • CalDav (Score:5, Informative)

    by jlittle (122165) <jlittle@cis.sta[ ]rd.edu ['nfo' in gap]> on Thursday November 29 2007, @12:38PM (#21520423) Homepage
    CalDav is the wave of the future, with most calendaring clients supporting it (but not MS), and many servers commercial and otherwise also supporting it (Zimbra). The real coming out party was the commercial release of both OSX Server 10.5 and the client, which have both ends. But guess what, the server is open source: calendar server [macosforge.org] can be gotten and put on any platform. If you want something today, Zimbra or OSX Server are there for the taking. RedHat has a Messenging product coming out based on Zimbra for this exact purpose.
    • Re: (Score:3, Insightful)

      by zoontf (64411)
      I just attended the Mac OS X Leopard Server seminar Apple is touring through the country with... in Boston... and during the talk about CalDAV, the Apple tech reps said that Microsoft had a plugin available to Office for using CalDAV to some degree. I don't know more than that, but at least some Apple people think that MS is on board with CalDAV. Actually, they listed the steering committee members for CalDAV on a big screen, and buried among the 50-some-odd names was Microsoft.

      So, CalDAV maybe worth more i
    • Re: (Score:3, Insightful)

      by wodgy7 (850851)
      CalDAV is indeed the holy grail. Finally we have something open source that supports all the major user-visible features of Exchange (time visibility, resource scheduling, etc.), is standards-based, and is supported by multiple vendors. There has been nothing like this for far too long.
  • by SuperKendall (25149) on Thursday November 29 2007, @12:38PM (#21520429)
    Apple provides a nice calendar server [ittoolbox.com] with Leopard server - but it works with Linux (any anything else running Python) as well...

  • Not really (Score:5, Informative)

    by initdeep (1073290) on Thursday November 29 2007, @12:39PM (#21520453)
    Zimbra http://www.zimbra.com/ [zimbra.com]


    Scalix http://www.scalix.com/ [scalix.com]


    are the two closest, but honestly, neither is a perfect replacement.

    • Re: (Score:3, Interesting)

      Zimbra is INCREDIBLY COMPLEX TO INSTALL. It was sold to Yahoo who essentially is going to abandon all open source and take it out of that market. Do NOT trust Yahoo on this one.

      Zimbra's programmers were daft. They would only make installs for certain releases of the OS and then they would get rude to those who were seeking support. They essentially created a product and abandoned those in the open source arena, and they don't care about you.

      Their install requirements, their installer script, and their a
  • Zimbra (Score:5, Interesting)

    by sg_oneill (159032) on Thursday November 29 2007, @12:42PM (#21520509)
    Zimbra pretty much does it all. The web client is top notch, and makes a perfectly fine outlook replacement (Yeah, I know. Just try it, seriously), and its got some serious scaling capacitys (Its used by some of the biggest ISPs around). Yahoo now owns it, so its got some name backing. The catch is the outlook compatible one ISNT so open source, but its pretty cheap.

    Citadels pretty nice too, and Ignatius foobar is a cool guy, but its a pretty eccentric product. I think they've kinda been fucked around a bit with outlook compatibility, but I admit I havent checked in a long time.
      • Re: (Score:3, Informative)

        Zimbra is, arguably, not Open Source.

        It comes under an 'attribution license' and hence cannot be forked. Its 'badgeware'.

        Ie: you cannot take the source code of Zimbra and produce your own version *without* the Zimbra logos.
  • by dominux (731134) on Thursday November 29 2007, @12:55PM (#21520763) Homepage
    in the mean time I am using webcalendar which works great. Lotus Domino runs on Linux and would be my preferred choice of proprietary solution, I am trying to get IBM to make Domino a CalDAV server, anyone who has an IBM rep is encouraged to beat them up about CalDAV support. www.bedework.org looks quite good now. Might have to re-evaluate that one.
  • iCal Server (Score:5, Informative)

    by HiredMan (5546) on Thursday November 29 2007, @01:00PM (#21520859) Journal
    Apple's iCal Server is Open Source PHP (with Twisted Framework) and based on the new CalDAV open standard. Everyone (with the possible exception of Microsoft) is moving to CalDAV as the open standard. Many big companies (Oracle, IBM, Google) are involved with the committee and hopefully the holy grail of inter-operable calendaring systems - including free/busy, invitations etc - is finally on the horizon.

    The server just officially went gold with Leopard but has actually been done for a while now. Apple's iCal Server and (closed source) Client are currently the most polished products but now that there is a solid CalDAV server I expect that the various clients with gain alot of polish and other CalDAV servers should start to roll out as well.

    Check out the CALCONNECT standards body for more information: http://www.calconnect.org/ [calconnect.org]

    =tkk

    PS Microsoft is finally a member but their commitment level is not that of the other partners.
    • Re: (Score:3, Informative)

      by Erskin (1651) *

      Apple's iCal Server is Open Source Python (with Twisted Framework) and based on the new CalDAV open standard.
      (It's probably an innocent slip of the brain, but figured I'd mention it for anybody unfamiliar with the tech so they don't get confused.)
    • Re: (Score:3, Informative)

      by MouseR (3264)
      It should be noted that

      - Apple's Calendar Server is indeed open source [royhooper.ca], unlike mentioned above.
      - Apple's current implementation of recurring events in their CalDav server has some issues.

      Disclaimer:
      It should also be noted that I work for Oracle in the very same division responsible for the calendaring stuff and that my intent is not to diminish Apple's offering. Oracle aims for full interoperability with other CalDAV-compliant offerings, including that of Apple.
  • Zimbra (Score:3, Interesting)

    by hackus (159037) on Thursday November 29 2007, @01:04PM (#21520921) Homepage
    http://www.zimbra.com/ [zimbra.com]

    We are replacing all of our Exchange users and dumping exchange by the end of the year.

    It is an open source free replacement for Exchange.

    Very nice and integrates well with Sunbird (Thunderbird Calander).

    -hack
  • by IGnatius T Foobar (4328) on Thursday November 29 2007, @01:15PM (#21521133) Homepage Journal
    You really want to check out Citadel [citadel.org]. It has a very comprehensive feature set -- not just calendars but also email, address books, message boards, instant messaging, access via all standard protocols plus a gorgeous ajax-style web user interface.

    The best part about Citadel is that it is very easy to install. There's an automatic installer script right on the web site. No fuss, no muss, just enter the install command and watch it go. No tedious mucking about with integrating all of the pieces yourself, as the entire Citadel system is self-contained.

    And the whole thing is GPL, unlike solutions such as Zimbra and Scalix which claim to be open source, but when you actually go there you find out that to get the full feature set you have to buy a commercial version. The Citadel project makes its very best work available to everyone on the same terms.
      • by IGnatius T Foobar (4328) on Thursday November 29 2007, @03:22PM (#21523163) Homepage Journal

        I've looked at Citadel myself. I think it looks good, but I also think that when you say "shared calendars" to most middle and upper management, it gets interpreted as "Outlook + Exchange".
        Sadly, there is quite a bit of truth to that statement. Fortunately, there is an Outlook connector for Citadel currently in beta test. It's quite nice -- while most non-Microsoft Outlook connectors merely do synchronization, this one implements a full store (i.e. the equivalent of what you get when you connect to Exchange). Outlook and non-Outlook clients will *both* be first class citizens on a Citadel system. We'll have this out within the next couple of months, and it'll be very useful.
  • by Arrogant-Bastard (141720) on Thursday November 29 2007, @02:25PM (#21522315)

    It makes my short list, along with ZImbra, Scalix, and Openchange (so far).

    The nice things about JES are (a) it's rock-solid (b) it works well with many mail clients, even horribly broken ones like Outlook (c) while it doesn't have every possible calendar feature in the world, it has all of the ones that people actually use (d) it scales amazingly well -- it's really no problem to get it to support millions of users (e) because it's been around for a while (including a prior incarnation as a Netscape product) there's a pretty solid support community for it (in addition to Sun) (f) it's flexible enough to support integration with other products.

    The bad things about JES are (a) the install is complicated, even if you're very accustomed to complex installs (b) the documentation, like much of Sun's documentation, is poorly written, verbose, uses opaque terminology, and lacks cohesion (c) the log files are inscrutable (d) it's somewhat bloated (somebody needs to trim all the legacy code out of it) (e) it's overkill for anyone who just needs a mail server (i.e., no calendaring).

    But...given that you get mail, calendaring, LDAP, all rolled up in one package -- it's at least worth looking at. I'm aware of any number of places that have migrated from Exchange to JES, so at least their requirements were met.

    • Re: (Score:3, Informative)

      by disasm (973689)
      opengroupware has a plugin for working with outlook. I haven't used that one in a while though.

      Sam
    • Re: (Score:3, Informative)

      by ahodgson (74077)
      I'm sure if Microsoft ever allows outside software to properly interoperate with Exchange, that will happen. But they don't.

      Shocking, I know.
    • Re: (Score:3, Insightful)

      by orclevegam (940336)
      It's also nice to be able to eliminate Windows machines off your network that you're not really using for anything but hosting a few select applications that require it. Having to have an entire server running just to host exchange server is a pain in the butt when you already have plenty of Linux servers around that could do the job if there was a cross-platform alternative to Exchange open source or otherwise.
    • by 99BottlesOfBeerInMyF (813746) on Thursday November 29 2007, @01:51PM (#21521729)

      s there some particular reason you need to replace Outlook for an Open Source alternative? This makes no sense to dump something that works and is clearly the best solution right now.

      Up until a few years ago, I would have agreed it was the best solution for most businesses, but times have changed. I don't know what industry you're in, but a lot of larger companies are introducing more Linux and Macs on their networks and the ability to function cross-platform and across a variety of clients is a huge feature for a lot of companies.

      Unless you just want to save a couple of bucks, there's nothing magical about an Open Source product that makes it better.

      According to MS, in order to license the current version of exchange it will cost you $4000 per server + $97 per user + some unnamed fee if you want to interconnect with other companies servers. So, assuming you have 1000 people and two servers, you're looking at over $100K. And for that price you can only use all the functionality if all your clients are on Windows, so your advertising people on Macs and your software development team on Linux both end up running their own little calendaring servers or using a shitty Web interface that has not kept up with the regular client. People with smartphones also end up costing you extra for connectors that allow them to access some of the functionality of your Exchange server, instead of all the functionality of a CalDav server.

      To summarize, the failures of Exchange are:

      • licensing costs
      • future licensing costs for upgrades to support new clients
      • lousy cross platform support
      • added expense to support smartphones
      • lack of choice for clients
      • lack of choice for server platform (only Windows and VMWare) Whereas CalDav servers like Zimbra also support OS X, Linux, Solaris, etc.
      • lack of choice for support and customization and services, only MS instead of RedHat, Zimbra Inc, IBM, etc. (If MS does not fix a security hole tht is a problem for you, you're screwed, whereas with CalDav you can hire someone else to fix it or even fix it using internal programming resources)

      ...there's nothing magical about an Open Source product that makes it better.

      Umm, not magical, but being OSS is a feature, one that Exchange is lacking. It is not the only feature that matters, but it does bring significant benefits, including reduced risk and protection from vendor lock-in.