[julienmarie]

It's mostly DNS records. Postal Server has good documentation and tools helping you set up quite everything in their UI.

It seems Stalwart also makes it easy in their UI based on the gif in the post. You can see at some point a list of DNS entries to update.

The only missing thing in all these is the PTR record which needs to be set up at the hosting level.

A good tool to check if everything is OK is https://www.mail-tester.com/