Example 18-5. expn

  #!/usr/bin/perl -w
  # expn -- convince smtp to divulge an alias expansion
  use strict;
  use Net::SMTP;
  use Sys::Hostname;
  my $fetch_mx = 0;
  # try loading the module, but don't blow up if missing
  eval {    
      require Net::DNS;
      Net::DNS->import('mx');
      $fetch_mx = 1;
  };
  my $selfname = hostname( );
  die "usage: $0 address\@host ...\n" unless @ARGV;
  # Find out whether called as "vrfy" or "expn".
  my $VERB = ($0 =~ /ve?ri?fy$/i)  ? 'VRFY' : 'EXPN';
  my $multi = @ARGV > 1;
  my $remote;
  # Iterate over addresses give on command line.
  foreach my $combo (@ARGV) {
      my ($name, $host) = split(/\@/, $combo);
      my @hosts;
      $host ||= 'localhost';
      @hosts = map { $_->exchange } mx($host)     if $fetch_mx;
      @hosts = ($host)   unless @hosts;
      foreach my $host (@hosts) { 
          print $VERB eq 'VRFY' ? "Verify" : "Expand", 
              "ing $name at $host ($combo):";
          $remote = Net::SMTP->new($host, Hello => $selfname);
          unless ($remote) { 
              warn "cannot connect to $host\n";
              next;
          }
          print "\n";
          if ($VERB eq 'VRFY') {
              $remote->verify($name);
          } elsif ($VERB eq 'EXPN') {
              $remote->expand($name);
          }
          last if $remote->code =  = 221;
          next if $remote->code =  = 220;
          print $remote->message;
          $remote->quit;
          print "\n" if $multi;
      }
  }