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Using SM15, it appears that messages with a null sender (MAIL FROM:<>) skip many of the spam checks (PTR check, greylisting, etc.). We have spammers exploiting this to deliver spam to SmarterMail servers.
It might even skip the SMTP Auth as the other day I saw a spoofed message where the From address was the same as the local domain. It was sent from an external IP address and got delivered to the user, but should have been denied because all users must authenticate.
Thanks for checking into this.