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:
"" postmaster@example.com
<> postmaster@example.com
Actually not sure which one works...