Our organization is running Exchange 2013 CU9 (On-Prem) (We also have some 2010 role servers due to retire, but still live), and all users have been on 2013 for ~4-5 months. About 1 1/2 months ago - a while after CU9 was applied, users started reporting instances in which they reply to an Exchange calendar invite in Outlook, and immediately get an email bounce. We did also create a new 2013 GAL around the time of CU9. Upon further exploration, the reply message generated by them clicking accept, etc. for the meeting is found to be in their outbox. If you open that message and click send, the message then sends with no problem, and the organizers calendar at that point reflects the acceptance. This did not appear to be happening for all users, so we tried to migrate one user from a mailbox database/server to another database/server. The issue appeared to be resolved. We later moved this user back to their orginal database/server and the issue did not return. Unfortunately, subsequent attempts at using the same fix steps have resulted in mixed results. For some users the issue remains/returns. Can anyone help suggest where to check for issues? 

Here is the content of the bounce you receive:

Hi Octavio,

Before going further, I'd like to ask the following questions to troubleshoot the issue:

Does the same issue also occur in OWA?

Have you tried to check this issue on another PC?

Best regards,