Just recently in the last week or so users have been noticing that they don't get new emails in their inbox unless they either manually update the folder or they close and re-open outlook.
I've confirmed this on a test account I have there as well. Outlook is a mix of 2010 and 2013 clients.
The only change recently made was we had taken the default exchange 2010 mailbox and split it out to 3 different mailboxes. Migrated all the user accounts to one of those 3 new databases, then migrated off the abritation accounts and removed the default database from within the exchange console. However, I wouldn't think that would be causing this issue. Also I should mention online mode seems to work fine and mail to mobile devices is fine.
I don't really see anything in the event logs to indicate what might be happening. A
Looking at the connection status of outlook, everything shows connected.