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 SaMoToP. For example Postfix has to be
tweaked like this:

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

$config_directory/generic:
	""	[email protected]
	<>	[email protected]

Actually not sure which one works...

I kindda know this whole stuff is written in completely wrong way, but I have
reasons for that. SaMoToP prevent me from creating a separate transport so I'm
just using MailDir and directly reading files from there in separate task. Also
socket permissions can't actually be hinted to SaMoToP, and I don't want to
mess with umask in this case, hence the extra task that changes socket
permissions later. The whole MailDir idea was shamelessly ripped from
https://github.com/meysam81/lori, kudos to the author. And if you think I
actually copypasted too much code - you are free to use this under APACHE-2.0
license.