Problem:
Mail server is running but login always fails.
Diagnosis:
# telnet localhost 143
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
* OK [CAPABILITY IMAP4rev1 UIDPLUS CHILDREN NAMESPACE THREAD=ORDEREDSUBJECT THREAD=REFERENCES SORT QUOTA IDLE ACL ACL2=UNION STARTTLS]
1 login test@testdomain.com testpassword
1 NO Login failed.
Solution:
This maybe caused by mail configuration in Plesk getting corrupted or not in sync. Run the following command as root to repair the configuration.
/usr/local/psa/admin/sbin/mchk --with-spam