I made an update to zmfilter.pl to enable use with an external mail relay that requires authentication. Thought others might find it useful, but wasn't sure how to go about passing it along.
Changed lines 1009 and 1103 in /usr/bin/zmfilter.pl to:
my @mail_credentials = split('\|', $Config{ZM_EMAIL_HOST});
MIME::Lite->send( "smtp", $mail_credentials[0], AuthUser=>$mail_credentials[1], AuthPass=>$mail_credentials[2], Timeout=>60 );
Then in Options->Email->EMAIL_HOST it looks like: mail_relay_address|username|password
This change will break the current mail_relay_address only so it shouldn't be applied if you are using localhost or an external relay that doesn't require authentication (do they exist?).