Hi Cuttergot,
It's stuck in the outbox because the Exchange server is severing the connection causing a time-out. Call up your IT company and ask them to adjust EWS and IIS settings.
Here's some proof that the issue is real and it is the Exchange server: http://stackoverflow.com/questions/10091745/exchange-web-services-attachment-loa d-is-slow
Here's more info:
http://msdn.microsoft.com/en-us/library/office/jj945066(v=exchg.150).aspx
http://blogs.msdn.com/b/mstehle/archive/2010/11/09/ews-best-practices-understand -throttling-policies.aspx
Let me save you some time... Since you're going to Outlook for Mac here's an article that you'll need to read to try and solve this same problem with Outlook for Mac, this is from the Outlook forum: http://social.technet.microsoft.com/Forums/en-US/1d1eadc2-db89-4147-bd9c-38a1435 1ce8e/slow-attachment-upload-with-exchange-web-services-outlook-2011-for-mac?for um=exchange2010
Also, look back at my previous posts, send that information to your IT company.
All,
Going forward, to help us troubleshoot, please include on the top line of your post/replies the following:
1) Version of Microsoft Exchange, if you know the SP and ** levels, please provide that information too.
2) Version of OS X? 10.9.1 I assume.
3) If you have Autodiscover selected or if you have to manually select your mail servers.
4) Is your Exchange Server hosted offsite or at your office?
5) Are you having issues while at the office and/or out of the office?