Perhaps it would be appropriate to add some language to section 3.7, where
the requirement to send an "undeliverable mail" notification message is
described. I'm thinking about something like this:
An SMTP server MAY decide not to send the "undeliverable mail"
notification message when it can determined that the original
message had malicious or deceitful intent. Determination of
such intent is beyond the scope of this specification.
This would be an accurate reflection of what many servers are doing today.