I am setting up a postfix/dovecot mail server with virtual accounts, per the Chris Haas tutorial. I'm finding that virtual users who also have local user accounts on the mail server cannot access their email remotely via IMAP. They're told they cannot login. (I'm using Thunderbird for that).

These same users can login when emulating IMAP locally via telnet.

Virtual users without local accounts have no trouble with IMAP access from remote clients.

These local user accounts have vestiges of prior efforts in their home directories: mbox files, Mail and mail directories.

I've looked at the logs for clues to where the remote login process is failing (dovecot authentication failure? confusion over where emails are stored?) but found nothing helpful.

I haven't found much in the dovecot or postfix documentation that describes the IMAP login process and expectations in enough detail to help me diagnose this.

So: how do I go about identifying the problem and researching a solution?