Dovecot has multiple auth socket types for different purposes: client, userdb and master. Admins sometimes configure the wrong socket to Exim, which currently simply causes Exim to hang until it reaches a timeout. Unfortunately due to some initial design mistakes the different socket types aren't especially easy to differentiate from each others in the code. But it is still possible, so this patch gives a nicer error message if wrong socket type is used:
authentication socket type mismatch (connected to auth-master instead of auth-client)
