RFC 2821 states (sections 3.3, 4.1.1.4 and 6.1) that the SMTP server
has two options: accept or reject the message, and has two options to
send back OK or Failure (5xx). HOWEVER, once the message has been
accepted as per this section (6.1):
" When the receiver-SMTP accepts a piece of mail (by sending a "250 OK"
message in response to DATA), it is accepting responsibility for
delivering or relaying the message. It must take this responsibility
[snip]
If there is a delivery failure after acceptance of a message, the
receiver-SMTP MUST formulate and mail a notification message. This
Therefore, the RFC does dictate what should been done with the message
once it has been filtered. Even though many anti-spam systems honor this
"in breach", this is another illustration of how protocols affect other
things, even filtering.
Absolutely, but of course it's not clear that any recipient or
administratively authorised disposition of the message can count as
"delivery failure". I'm not sure that silent failures due to filtering
*are* contrary to this recommendation.
Some say it's better for rejection to be earlier and explicit, but
successfully delivering to /dev/null isn't prohibited anywhere I can think
of.
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