I use this piece of code to check if the domain part of the e-mail address is valid. 
$email = "user\@perlmonks.org";

($username,$domain) = split(/@/,$email);

use Net::DNS;
$resolver = Net::DNS::Resolver->new();

$response ||= $resolver->query($domain,"MX") || $resolver->query($doma
+in,"A");

defined ($response) ?  print "$domain is valid\n" : print "$domain is 
+invalid\n";

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In reply to Validate domain in e-mail addresses by gordan

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