Subject: McAfee Spamkiller 2.1 for Domino mail servers.
Are you using Domino Web Access when you send the email?
I have discover that almost every email we send, using Web Access version 6.5.1 are quarantined. There are several rules involved, including MISSING_SUBJECT.
The workaround is to change mail format in Web Access preferences, from HTML to Plain Text. Then it works.