The exact error message could be obtained by using below command – tail – f /var/log/maillog
Cause: It may be because of the dovecot indexes are corrupted.
1. Delete all files named “dovecot” in the path “/home/user/mail/domainname.com/user” using rm -f dovecot*
2. You can simply rename the dovecot.index page as : mv dovecot.index dovecot.index_bk
Try to access the email that had the error, should have returned to normal.