First of all make sure to never listen on public IPs and always put sockets in
directories not acessible by just about everyone on your system. Though
listening on 127.0.0.1:XXXX is equivalent to creating a world-writable socket.

To catch bounces (so they wouldn't stuck in upper mail server) make sure sender
envelope address is real as required by mail library (actually not sure whether
this applies to mailin). For example Postfix has to be tweaked like this:

$config_directory/main.cf:
	smtp_generic_maps = hash:$config_directory/generic

$config_directory/generic:
	""	postmaster@example.com
	<>	postmaster@example.com

Actually not sure which one works...

PS: If you are reading this on GitHub - it's a RO mirror from original repo:

http://fs.b1t.name/smtp2tg/timeline
