command to debug user and mailbox or shared mailbox issues
doveadm acl debug doveadm acl debug -u user@domain Public/mailbox
authenticate users against unix system accounts
authenticate users against virtual (non-unix system) accounts
hybrid of unix system accounts and virtual users
Dovecot supports defining multiple authentication databases, so that if the password doesn't match in the first database, it checks the next one. This can be useful if you want to easily support having both local system users in /etc/passwd and virtual users.
resources
Dovecot Mail List: http://www.dovecot.org/mailinglists.html