saslauthd

About
saslauthd is a SASL authentication daemon. It works with Dovecot and Postfix but is (as far as I can tell) provided separately.

Configuration is in /etc/default/saslauthd.

Testing
testsaslauthd -u username -p password

This should be a system username, rather than a dovecot virtual username, if Dovecot is configured for virtual users.