Fighting spam via greylisting

Felix M. Palmen zirias at
Wed Apr 25 12:08:48 PDT 2007

* USM Bish <bish at> [20070425 20:43]:
> IIRC, usually  greylisting involves sending Err Code  451 (viz
> Requested action  aborted: local  error in  processing). Since
> this implies that  ATRN request cannot be processed  now, or a
> temporary failure, most modern MTAs resend the mail on receipt
> of Err 451.
> OTOH, Err Code 450 is  "Request mail action not taken: mailbox
> unavailable" (which is  perhaps what you mean  by saying "user
> not active now"). This is  a serious error, and indicates ATRN
> request  has  been *refused*.  MTAs  compliant  with RFC  822,
> should not respond this.

That's wrong. Every 4xx code is considered a temporary error, and
sending MTAs /should/ retry, but they are not required to do so.

Only the 5xx codes are used for permanent errors and indicate that the
sender must not retry the same transaction.

Regards, Felix

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