I like to tinker with new scripts/programs, and I also like to figure out how to do things properly, or at least as proper as can be through reading the manuals and browsing forums/guides.
So this is one thing I had to figure out in order to allow my servers to send e-mail that wouldn’t get bounced back through the various providers, and my goal was to accomplish this, without using SMTP authentication to my Mailcow server in order to send e-mail.
For example, this is how you would configure postfix to relay e-mail to G-Mail.
I would do a similar type of setup for my Mailcow server, instead of sending/authenticating with G-Mail, I would send my mail directly to my mail server and authenticate that way. While this solution would work for me, I just didn’t like the way it was handled.
This guide is how I accomplished my goal of sending e-mail through my various servers, to relay safely to my Mailcow server. I take no credit for the majority of what is listed here, I merely collected information from various other sites/guides, I will do my best to link to the original guides I used to ensure they get credit.
This guide assumes you have a fully working Mailcow server, with all configurations and DNS completed. It also assumes you are running at least Debian 11. I am running Debian 11 on all my servers, again the commands for other versions of Linux variants are likely similar, but packages/paths are going to be different.