Switching Mail from Microsoft to a centos Linux server

After switching back to our Centos Server when we try to access email accounts…
We get the following error.
Unexpected condition from IMAP server, closed or corrupt connection to IMAP. Possible mailbox corruption.

We have restarted the services that are recommend on the server.
We have rebooted the server.
webmail.domainname is accessible but wont allow login.