SMTP server error message

[Eli Zaretskii]

Hi,

I need some help from the SMTP experts here.

Since about a couple of days ago, some of the messages I send via
smtpmail fail because the SMTP server sends this response, after the
message is sent to it:

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

When this response comes in, smtpmail bails out.  This only happens
with about 25% probability, and in particular if I immediately resend,
the message is sent with no problems.

What does that response mean, and what can I do to avoid it (apart of
hacking smtpmail)?

TIA