Forwarding only bounce messages to a different SMTP server

emailemail-bouncespostfixsmtp

I have a Postfix SMTP server that sends transactional emails from a web service. These messages use VERP for the return path, so bounces go back to an address like this:

bounce-7232-useremail=userdomain.com@e.mydomain.com

This postfix server running on e.mydomain.com is used exclusively to send email, there are no local mailboxes, POP or IMAP access, and so forth. Only systems on the local network can relay mail through it.

I then have a separate custom SMTP application that only processes bounces running on the same server (e.mydomain.com), but on a different port (8025). It drops any messages that aren't going to a properly formatted bounce address. Emails with properly formatted bounce addresses are accepted.

When a bounce is accepted, this custom application looks up the proper user in the database based on the bounce email address, and increments a bounce counter. The main web service will only send transactional email to users who's bounce count isn't over a threshold.

My questions are these:

Would it be better to set up my bounce handling SMTP (bounces.mydomain.com) server to handle bounces directly (and run on port 25)? Or is it better to have all bounces go to my postfix server, and then have postfix forward only the bounces to the bounce SMTP application?
If it is better to have postfix handle all incoming messages, how do I configure it to forward only messages formatted like the above address to another SMTP server, running on an unprivileged port (8025)?

Best Answer

I am always reluctant to put services on ports that they don't usually live-- not because of a software deficiency, but because of people deficiencies. A sysadmin who inherits this setup has to be pretty good to track down the architecture of "where emails go when they bounce", or else your documentation needs to be pretty clear (and easy to find).

So, to answer your first question-- my suggestion is to have a separate server that handles bounces. This makes things nicely documented via dns, instead of buried in a config file for postfix.

If you choose to ignore that advice, utilizing the transport maps of postfix will allow you to do so. For example, adding this to main.cf:

transport_maps = regexp:/etc/postfix/transport

and using something like this in your transport file:

/bounce.*/      smtp:bounces.mydomain.com:8025

(don't forget to 'postmap /etc/postfix/transport' and 'postfix reload')

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Postfix is extremely flexible (and therefore, complex) in its configuration, so there are various ways to achieve this. The simplest way would probably be to use a transport(5) table.

First, enable the use of a transport table in postfix:

/etc/postfix/main.cf:
    transport_maps = hash:/etc/postfix/transport

You'll also have to make sure that Postfix accepts mails for the addresses that will be handled by Lamson. Have a look at permit_auth_destination for the rules Postfix will apply to determine valid recipient addresses. For the following example, assuming "example.com" is a domain not otherwise known to Postfix, it's probably easiest to simply add it as relay domain:

/etc/postfix/main.cf:
    relay_domains = example.com

Then, create an appropriate table. E.g. to redirect all mail for the domain "example.com" as well as mail for "user@mydomain.org" to your local Lamson listening at port 10025:

/etc/postfix/transport:
    example.com          smtp:127.0.0.1:10025
    user@mydomain.org    smtp:127.0.0.1:10025

After that (and then once after every update to the transport table file) don't forget to run:

$ postmap /etc/postfix/transport

This should get you going. Be sure to read the transport(5) man page, which will give you more ideas on how to use this powerful facility.

How to “filter” postfix-generated bounce messages

In this case you do it too complicated. This is the easy way.

You need to have this (additional) in your main.cf:

notify_classes = resource, software, bounce, 2bounce
bounce_notice_recipient = random.bounce@example.com
2bounce_notice_recipient = random.bounce@example.com
transport_maps = hash:/etc/postfix/transport_maps   #or another file. See below.

in the /etc/postfix/transport_maps put

random.bounce@example.com    smtp:127.0.0.2:25

then postmap /etc/postfix/transport_maps. Afterwards postfix reload or restart Postfix.

This will notify the random.bounce@example.com user of all bounces and as this user is said to be transported through 127.0.0.2:25 you get all these mails through that server. Replace the addresses/IP/port according to your environment.

Best Answer

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