Guerrilla SMTPd and PHP SMTP servers

Ever since I took a look at the implementation of Kill the Newsletter! I have been thinking of recreating it with PHP.

Guerrilla Mail before switching to Go were using an SMTP server implemented in PHP called Guerrilla SMTPd.

The purpose of this daemon is to grab the email, save it to the database and disconnect as quickly as possible.

Maybe this could serve as an inspiration, as the code is quite old, and I'm not even sure if it's running on PHP 8.1. I think a modern take on Guerrilla SMTPd could easily lead to the world of async PHP, Swoole, Fibers, ReactPHP, etc.

The other projects I found looked half-baked or claimed questionable things.