the word your looking for is smart host, you would setup your mailcow to smart host to your VPS. The VPS would then send out to the destination mail server.

The VPS will need to have SPF/DKIM setup for it to not end up on block lists

Hi,

I have Mailcow setup on a local machine and want to relay it's outgoing mail through a VPS. 
My local IP is on some blacklists, while my VPS is not.

Despite extensive googling I was unable to find how I should be configuring the SMTP relay on the VPS, all instructions seem to be for pointing postfix to some existing SMTP relay.

What's the easiest way to setup my own SMTP relay (ideally dockerized)? Am I correct in assuming that I need to redo DKIM/DMARC on the VPS?

Host SMTP relay?