On Wed, Feb 07, 2024 at 07:59:44AM -0500, John Hill via Postfix-users wrote:
> Do mail servers as a whole stop sending an email after a few errors?
For a single message, surer
On soft errors (4XX), most retry, typically stopping after a maximal
delay. The retry strategy varies, but 4,000 retries in one day is
atypical, since that averages out to retrying approximately every 20
seconds, without backoff.
Sendmail, and IIRC Exim, retry at fixed intervals, with IIRC 30minutes
being somewhat popular. Postfix performs exponential backoff from an
initial (default 300s) minimal backoff time to a (default 4000s) maximum
backoff time.
> I have a server I have blocked in my firewall. It continues to try and is
> blocked as many as 4000+ times a day.
If you drop packets from a system that has multiple messages to send,
then none of the messages will be rejected outright, and they'll all
queue up. Once there are enough messages queued, you can easily see
thousands of TCP SYN attempts a day because:
- Even a single application-level connection attempt will send
multiple TCP SYNs until the connection times out.
- Each message will be retried periodically.
- Meanwhile more messages can arrive, or be already queued.
> If postscreen was set to deny it, would that signal the server and limit the
> attempts?
With a legitimate MTA, SMTP-layer reject would indeed be more effective.
(But then perhaps you should not be rejecting the mail???).
--
Viktor.
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