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Some postfix notes

Posted by jpluimers on 2020/10/15

Postfix has documentation on primary MX and secondary MX, but not on tertiary MX.

If the primary MX is down, you have a series of secondary MX and tertiary MX that configured the same way, MX DNS priority for primary, the series of secondary MX and tertiary MX have increasing numbers, and the primary MX goes down, then senders can get “too many hops” as secondary and tertiary MX are looping.

I had a hard time finding a good and easy solution as these queries do not return many meaningful results:

Here are some links that helped getting this solved:

  • [WayBack] Postfix Frequently Asked Questions: What does “Error: too many hops” mean?

    Short answer: this message means that mail is probably looping. If you see this after you turned on Postfix content filtering, then you have made a mistake that causes mail to be filtered repeatedly. This is cured by appropriate use of content_filter=, header_checks=, and body_checks=.

    Long answer: the message has too many Received: message headers. A received header is added whenever Postfix (or any MTA) receives a message. A large number of Received: message headers is an indication that mail is looping around.

    Side comment: email uses the opposite of the technique that is used to avoid IP forwarding loops. With IP, the sender sets a TTL (time to live) field in the IP header. The field is decremented by each router. When the TTL reaches zero the packet is discarded and an ICMP error message is returned to the sender.

  • [WayBack] Error: too many hops (in reply to end of DATA command) · Issue #713 · mail-in-a-box/mailinabox · GitHub

    In case you or anyone else was/is wondering about the mydestination = localhost thing, the reason it has to be set to just localhost is because MIAB uses Postfix’s “virtual domain hosting” (http://www.postfix.org/VIRTUAL_README.html) support. Per the documentation for mydestination at http://www.postfix.org/postconf.5.html#mydestination:

    Do not specify the names of virtual domains – those domains are specified elsewhere. See VIRTUAL_README for more information.

    (in the context of MIAB every domain is a virtual domain).

In my case a series of these:

Received: from mwgp.xs4all.nl (mwgp.xs4all.nl [80.101.239.92])
    by fiber24315337242.heldenvannu.net (Postfix) with ESMTP id 26395200FE
    for <jeroen@pluimers.com>; Fri, 29 Jun 2018 11:01:02 +0200 (CEST)
Received: from fiber24315337242.heldenvannu.net (unknown [37.153.243.246])
    by mwgp.xs4all.nl (Postfix) with ESMTP id 077A5E937
    for <jeroen@pluimers.com>; Fri, 29 Jun 2018 11:01:02 +0200 (CEST)

Specifying the transport will likely help me solve this problem:

This all came down to editing /etc/postfix/transport adding lines for each relayed domain like this one:

example.org    smtp:[mx-a-record.example.org]

Lines like it direct to use the smtp transport and use a specific host (normally, the relay transport is being used).

After this:

# postmap /etc/postfix/transport
# rcpostfix reload

I choose not to configure [WayBack] Postfix Configuration Parameters: relay_recipient_maps, but might if I had an automated way of replicating lists of valid (and invalid) users.

Another option was confirmed at [WayBack] Software-update: Postfix 3.4.0 / 3.3.3 / 3.2.8 / 3.1.11 / 3.0.15 – Computer – Downloads – Tweakers by [WayBack] menocchio. Thanks!

Dat is volgens mij eenvoudig op te lossen met relay_transport of transport_maps. Zie ook: Postfix transport table format.

Daarmee dwing je de secondary servers de mail altijd af te willen leveren bij de primary server (en dus niet bij een andere secondary). En als de primary niet online is, dan wacht ie netjes tot dat wel het geval is :-)

Bijvoorbeeld:
relay_transport = smtp:[primarymx.domain.tld]

Likely relevant: [WayBack] The Book of Postfix

Maybe relevant in the future:

Found on my hunt for the above:

Try not to make typo’s: [WayBack] postfix appears not finding MX records or host names from DNS

Interesting thought, but not sure how smart SPAM bots are now: [Archive.is] Spam relaying through secondary MX… – Google Groups

To archive this:

  1. Rename from
  2. To
  3. Then save in Archive.is

–jeroen

This entry was posted on 2020/10/15 at 18:00 and is filed under *nix, Communications Development, Development, DevOps, DNS, etckeeper, Infrastructure, Internet, Internet protocol suite, Linux, Power User, SMTP. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

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