Whilst we do provide TurnKey configured to send mail directly from localhost as you note, it's not really what we recommend.

We generally recommend that users use a third party SMTP relay service for sending mail. The reason why we recommend that is because sending emails reliably is hard! It's hard enough to ensure that emails even arrive at their intended destination, let alone not getting tagged as spam. To make configuring Postfix to send mail via a remote SMTP relay easy, we provide a mail relay Confconsole plugin. So personally, my recommendation would be to configure all of your servers to send via a remote SMTP. I would personally use the same remote SMTP for sending all mail from all servers.

Having said that, if you are determined to send emails from your own server, and configure that as your own STMP relay, then it should be possible. TBH, I have no idea exactly how to do that, but it should be possible. A quick google turned up a Postfix doc page that looks relevant - although TBH, I didn't read it at length, so I'm not sure how much detail it goes into. If that isn't as good as I think it is, perhaps consider browsing the rest of the Postfix docs and/or Postfix FAQ. If all else fails, perhaps post on an appropriate Postfix mailing list (I assume that "postfix-users" is probably the right one?! If you do post on their mailing list, then please ensure that you note you are using the Debian package of Postfix. To get the specific version of Postfix installed, either run 'apt policy postfix' or 'postconf mail_version'.

Once you have set that up, hopefully you should be able to use the "custom" configuration of our Confconsole mail relay plugin to configure your other servers to use your own SMTP relay server.

Regardless of how things go, please let us know how you progress. No promises, but if you hit any issues, perhaps I might have some further input?