Symptoms
Being unable to send or receive mail and receiving errors like the following in the Exim error log:
2015-03-05 11:43:41 fixed_login authenticator failed for (ispip.net) ([XX.XX.XX.XX]) [YY.YY.YY.YY]: 535 Incorrect authentication data (set_id=user@domain.tld)
Troubleshooting
535 Incorrect authentication data is typically caused by incorrect login details. If the issue continues after confirming the login details, Dovecot debugging can help find the cause: