Why doesn't Dovecot work?
Dovecot always logs an error message if anything goes wrong, so make sure you're looking at the correct log files. Debug messages may be written to a different log file than error messages. See ["Logging"].
Try logging in manually by sending IMAP commands. If you're trying with an IMAP client you can't be sure if the problem is with the client's configuration or Dovecot's configuration. See TestInstallation (or TestPop3Installation).
It says password mismatch
If you're using PAM:
Make sure that dovecot-auth is running as root (assuming it's using /etc/shadow).
PAM errors aren't written to Dovecot's own logs. Usually they go to /var/log/auth.log or something similar.
- Unfortunately PAM's error messages aren't always all that helpful in figuring out what exactly the problem is.
Usually the problem is that you don't have a correctly named file in /etc/pam.d/. See ["PasswordDatabase/PAM"]
For non-PAM set auth_debug_passwords=yes and look at what it says in the logs. If it's trying to use a wrong password scheme, you can change that. See ["Authentication/PasswordSchemes"]