It seems it's only usable as a forwarder of mails sent from external (WAN) to one internal SMTP Server:
http://doc.pfsense.org/index.php/Postfix
Postfix forwarder package at pfSense is a MTA configured to be a part of a full antispam mechanism. No local accounts are configured in this package, all messages are received, filtered and forwarded to your internal mail server.
Sorry for bugging.