Answer1. There is no way a AntiSpam will even try to go into the application header and identify if the Application is SMTP. The only 1 way for the AS to identify the packetsl and that is to look at the Transport layer header and then port number.
AS send a request to the Sophos Antispam Server and The erver tells the SRX if the sender's domain is a Spam sender. Sophos server is non configurable by an SRX user. So this matter can be taken up with the Sophos if they can provide such facility. Nice creativity though!!