Michael Deutschmann wrote:
Case 3: Sarah's message got SPF neutral, example.net offers SWK-SPF:
MAIL FROM: <> AUTH=fred(_at_)example(_dot_)org
Case 4: Sarah's message got SPF neutral, example.net doesn't do SWK-SPF:
MAIL FROM: <shamsrs-fred(_at_)example(_dot_)org>
In the latter two cases, a bounce may be needed to learn that a recipient
mailbox has been deleted. It is pretty useless to keep an obsolete recipe
A mail domain that does not in-transaction reject when the mailbox
doesn't exist deserves to lose. (They can wait until DATA if they want to
defeat stealth VRFYs.)
They may accept an RCPT if they have a forwarding recipe for it.
If there is an in-transaction rejection in Case 3/4, the forwarder will
have to shut down the input end of the forward anyway, to protect itself
from an accumulation of deadletters.
Does that imply no mail queue? In that case, how can it handle the case of
multiple recipients requiring multiple connections, one of which fails
after DATA? (The raison d'être of LMTP.)
Procedures for this will already
need to be in place, to deal with users who fail to whitelist. These
will have the effect of terminating the forward if the recipient cannot
be contacted.
Except "no such user", 5xx replies may imply the recipient will still be
valid at some future date. It is always difficult to recognize those cases
properly.
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