On 14 Oct 2019, at 14:20, Thomas Walter via mailop wrote:
Of course I don't have the experience in the last category, but I'd
like
to learn. Why can't you reject emails post-DATA?
Is it a performance issue? Google or Bing find 935.000.000 search
results in 0,60 seconds for the word "spam", but they can't do a spam
check in that amount of time?
This is not a pure performance issue. It's more a matter of not having
the data at hand to decide whether the message is ham or spam. To do so,
filters need user feedback.
In order to have the data, a trickle of those suspicious messages are
allowed to get in. What the recipients do with them is used to decide
what to do with future messages.
Protocol-wise, what is a sender supposed to do with a post-DATA
rejection? Is that rejection associated to one of the RFC-5321 RCPT TOs?
All of them? None, because it's actually a content issue? What if the
policies for each recipient differ?
SMTP does not allow for this level of granularity in the signaling.
MTAs know how to deal with a post-RCPT rejection. A post-DATA is an
entirely different thing.
There's also the option of sending a NDR after accepting the message,
which is undesirable for a plethora of other reasons.
Best regards
-lem
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