[Mailman-Users] DKIM signing issue - relaying mailman e-mails from third party sources

[Dlugasny]

Hallo Mark, Kolleagues,

maybe somebody will be able to help me here.

I would like to relay (also check with Antivir and sign them with DKIM) all Mailman e-mails from our external partners to our final customers. Relaying seems to work nice but we have a problem with DKIM authentication and Return-Path.
Mailman server needs to stay by external partners with old domains and needs to receive bounces. My SMTP gateway is only responsible for receiving and sending e-mails out from the all mailman instances.

The problem is that we are sending an E-mail which looks as follow:

From: campaign at myserver.com
Return-Path: mailman-bounces at external-company.com
To: @gmail.com

The problem is that DKIM check on the gmail server server (and all others) returning error:
[...mailman-bounces at external-company.com](mailto:mailman-bounces at external-company.com) does not designate xx.xx.xx.xx as permitted sender

How to solve that issue ? At the moment we have a DKIM key only for myserver.com. Why DKIM check checking Return-Path and not From address ?

Could You please help here how to manage that issue ? We simply would like to forward all messages from the external mailman instances installed on the different domains to the final customers using our sender domain [myserver.com.](mailto:campaign at myserver.com)

I will appreciate any feedback from Your side.

Cheers
Dlugasny