If you're unable to create a user for mtn, this example shows an
alternative, using procmail.

(all this applies to both NodeA and NodeB, except commandline arguments and $recipient)
 
copy _forward to ~/.forward
copy _procmailrc to ~/.procmailrc
touch ~/mtn-drop

edit conf.pl and set:
    $recipient = "you\@fooisp.net" ; # or "you\@blocked.net"
    $maildrop = $ENV{"HOME"} . "/mtn-drop" ;
    $clear_maildrop = 1 ;
    $debuglevel = 0 ;

on NodeA (blocked by firewall), run:
./mtn listen 6001

on NodeB (on-the-net), run:
./mtn forward foobar.foohost.net 6667




All mail received by either host with "--MT--" in the subject line will be
stored in "~/mtn-drop". All other msgs will be stored in your default
maildrop ($MAIL). So, this is the safe way of operating mtn, which requires
alil more hacking.


Enjoy.
