Rejecting based upon RCPT TO alone is a significant logical hole it allows
automated address harvesters to poll an MTA with random addresses so that
it can build up a list of valid ones.
Do you have any evidence that this is taking place?
My impression is that such dictionary attacks for address harvesting may
have been a tactic in the late 90's, but the volume of spam today seems to
be running completely open loop, with spammers not bothering to check which
names are delivered/rejected, not cleaning their databases, but just
sending volumes blindly.
And, even if there is address harvesting taking place, since when is having
the address of a valid recipient sufficient for delivery?
Len
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