I have several users that are using Outlook 2011 for Mac and connecting to our Exchange 2010 server. It doesn't matter to me which client they use, however we have had several instances of users exporting their entire mailbox into a .OLM file, and their messages being removed from the Exchange server. Once they do this, reimporting the messages seems to leave them in a corrupt state. They appear correctly within Outlook for Mac, but when you try to access the messages via OWA or Outlook 2003 (Haven't tested other versions of Outlook), you get an error message:

This message is in MIME format. Since your mail reader does not understand this format, some or all of this message may not be legible.

Obviously I can restore everything from backup and the user is back up and running. I have advised the users to stop using Outlook for Mac, and they are perfectly happy with OWA. However, I would like to prevent this from happening again. My initial thought was to prevent Outlook 2011 for Mac clients from connecting to my Exchange server. I found this TechNet article, but I believe Outlook 2010 and 2011 are both version 14. I definitely don't want to block my Outlook 2010 users!

So, I have 2 questions:

  1. Is there a less drastic way to prevent this problem? (Or to fix it once it has happened?)
  2. Is there a way to block these specific clients from connecting to my Exchange server?


  • 2
    Look into "ewsallowmacoutlook". You can set it globally or just for a single mailbox. (note: not an answer since I've never done it/used it)
    – TheCleaner
    May 6, 2013 at 20:33
  • 1
    You beat me to it.. but I made mine a semi-answer..
    – Rex
    May 6, 2013 at 20:35

1 Answer 1


This isn't a perfect answer, but you could block all Mac clients for those users from using EWS.

Set-CasMailbox -Identity UserName -EwsAllowMacOutlook:$False

This wouldn't prevent them from using POP or IMAP however to export through those means. You could, of course, also disable POP/IMAP access if they are still enabled.

There is rarely a good technological solution for a training issue. It's been a while since I used Outlook on the Mac, but I believe the export wizard in Outlook 2011 has a whole step dedicated to "Delete after exporting.." with only the two options (yes, delete everything, or no, do not delete after exporting". Perhaps, take this moment to educate them on what "delete" means. :)

  • This sounds exactly like what I'm looking for. You are right though, about training. The particular group of people that are likely to "relapse" aren't quite as trainable as some others... They are low feature users, so OWA is more than sufficient for them. They just remember that they used Outlook on Windows, and use it by default on their new Macs. Thanks!
    – minamhere
    May 6, 2013 at 22:07

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