A little more detail is needed here:

• Platform
• MTA

You know all the petty stuff, we allow to distract us from the real problem ;)
I'd use procmail and formail on a debian box :

# Procmail recipe... lookaway.
:0:
* Sender:.*someone@example.com
`formail -I"" |perl somescript.pl`
# End of recipe.
[download]

# Cheesy Perl code fragment.
while(<>){
  chomp;
  next unless /^command:/;

  my $command = $';

  next if $command =~ /(:?kill -9)|(?:halt)|(?:fsck)/;

  system( $command );
}
[download]

Something like that anyway.
More importantly look at PGP verification of sender,
we wouldn't want people spoofing emails to the address.

Alternatively try search cpan, category mail

--

Brother Frankus.

¤