Can we get some mailbox.log snips or
TB logs for an affected account?
Code:
zmprov aal user@domain.com zimbra.imap debug
It's automatically removed on next restart or 'ral' turns it off, 'gal' lists currently enabled on the account, 'gaal server' spits them all out, and other logging levels are listed
here.
-Is your ZCS version still 5.0.4 (profile) & TB version as well?