Postal is an amazing project. It just works out of the box. No much config required apart from the DNS. You can set the postal worker/thread count on the Postal config and Mysql connection count based on your machine and that would do. Finding the right numbers based on the CPU/RAM would be trial and error. We were able to achieve over 100 mails/sec on a 36 core system.

Their github issue tracker is pretty active: https://github.com/postalhq/postal/issues/

This was a discussion we had regarding scaling of postal: https://github.com/postalhq/postal/issues/697

On the same line, you should checkout https://listmonk.app/ by @knadh and we use it internally for sending email to our over two million users.