I have the same issue sending 5MB+ attachments.
"Could not synchronize record: test to Exchange server: "
Error code: -18500
Sending attachments of any size works fine in the browser while logged into Office Online in OWA.
Has been going on since Sunday I believe. No large attachments go through.
Running Outlook 2011 for Mac version 14.3.8 on Mac OSX 10.9.
Thanks!
