SMTP server error message

[Stephen J. Turnbull]

Eli Zaretskii writes:

 >   451 4.0.0 Spam filter end message failed - server - license [slot 1]

Greylisting.  The idea is that a real MTA will retry (it's a temporary
failure, which means you are being invited to try again later with
exactly the same address and message data), while a spammer will just
move on to the next victim's address.  Reduces spam by about 75%, but
most MTAs wait about 4 hours to retry, which is a long time for a list
like emacs-devel.

The greylisting MTA normally whitelists you immediately for a period
of time (typically 24 hours) after the first (temporary) failure.
This may explain your 25% failure rate, the others are within the
auto-whitelist period.

You don't say which SMTP server you're talking to.  If it's an
outbound relay, you might talk to the sysadmin and see if your host
can be permanently whitelisted.