This is kindda a HUGE rework. SMTP backend switched from SaMoToP to
mailin-embedded, resulting in:
 * LMTP support dropped (I hope just temporary);
 * no need to write anything to files as whole processing is done in memory;
 * this also means we can return status - whether message was sent, was there
   any errors etc, no internal queue exists now, SMTP server should handle
   queueing from now on;
 * possibility to deny mail to unknown/unconfigured recipients.