sub mail_verify {
    my $addr = shift;
    # copied code from Mail::Sendmail begins here
    my $word_rx = '[\x21\x23-\x27\x2A-\x2B\x2D\x2F\w\x3D\x3F]+';
    my $user_rx = $word_rx         # valid chars
             .'(?:\.' . $word_rx . ')*' # possibly more words preceded by a dot
            ;
    my $dom_rx = '\w[-\w]*(?:\.\w[-\w]*)*'; # less valid chars in domain names
    my $ip_rx = '\[\d{1,3}(?:\.\d{1,3}){3}\]';

    $address_rx = '((' . $user_rx . ')\@(' . $dom_rx . '|' . $ip_rx . '))';
    # copied code from Mail::Sendmail ends here
    # now use the regex:
    $addr =~ /^$address_rx$/;
}