Using older versions of Outlook for Mac in business environments
When you see the message that Sync is Pending it just lets you know that there will be changes made during the next sync. That doesn't mean there's a problem, it just lets you know that changes are pending.
This is not accurate.
Once you get a Sync is Pending you have to restart Outlook at a minimum sometimes restart the Mac and often just have to give up and go back to v2011 or use a web version. And before you try to blame the exchange version (which no user using Outlook has any influence over anyway) my companies is 14.3.123.0 according to OWA which I think is 2010 SP3.
This is a bug with the Outlook 2015 and not with the Exchange server, the user or anything else because as everyone as indicated the same account will work on Windows Outlook, OWA or even Apple mail with no problem.
Would also add that there is a reason this thread has 2800+ views and counting vs all the others with < 50 in the forum.