libevent may give a good performance boost: http://php.net/manual/en/ref.libevent.php
Docs not very clear, but assume that we would need to re-write the code to use PHP streams instead of PHP sockets

http://cr.yp.to/immhf/envelope.html
Need to add envelope headers to each received email. "Received:" and "Return Path"

{"error":true,"reason":"You have exceeded the rate limit. Please limit your requests to 20 per minute max. Thank you."}

This error is now on site

Is it possible to connect with any email client? IMAP or POP3?

Guerrilla Mail is no longer maintaining this project due to being replaced by our brother SMTP server project https://github.com/flashmob/go-guerrilla
If you're interested in taking ownership, please drop us a line

Seems like that's the most PHP can do right now. (See the TLS issue)

Golang seems to offer non-blocking asynchronous I/O built in to the language, but much cleaner because it doesn't use callbacks but lightweight threads and channels instead.

It also has TLS support, and possible to issue a STRATTLS command and easily upgrade the connection to TLS.

It was never the intention to stick to any particular language for this server. Actually, the server didn't use classes because there was a possibility that the server would be re-written to C one day.

The server in Go will be the 3rd major version of Guerrilla SMTPd. It is currently being tested on Guerrilla Mail, and will be released to here soon.

SMTP has STARTTLS extension

example from https://bugs.php.net/bug.php?id=36445

(Note, for non-blocking, it may return 0. keep on calling until a result is true or false)

!/usr/bin/php5