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if ($toaddress =~ /(\S+)[@]yourdomain|yourotherdomain/) {
   $user = $1;
...
   warn "Unknown user:  $toaddress\n";
   $user = 'postmaster'; # Or whoever should get slop.
}

# This is untested.
@users = map {
...
  warn "No users.\n";
  @users = ('postmaster');
}

  my $filename = "attachment"; {
    my ($y, $m, $d) = (localtime)[5,4,3];
...
  binmode FILEHANDLE; # Probably not necessary on *nix.
  print FILEHANDLE $content;
  close FILEHANDLE;

$;=sub{$/};@;=map{my($a,$b)=($_,$;);$;=sub{$a.$b->()}}
split//,".rekcah lreP rehtona tsuJ";$\=$ ;->();print$/