#!/usr/bin/perl -w
use strict;
use Mail::MboxParser;
my $mbox = '/var/spool/mail/www37';
my $mb = Mail::MboxParser->new($mbox, decode => 'ALL');
print "Total messages: ", $mb->nmsgs, "\n";

# iterating through the mailbox
while (my $msg = $mb->next_message) {
        my $body = $msg->body($msg->find_body);
        foreach my $line (split /\n/, $body) {
                my (undef, undef, $a) = split / /, $line if $line =~ /Host unknown/i && $line=~/^550/;
                if ( defined $a ) {
                        $a =~ s/^<(.+)>\.\.\.$/$1/;
                        print "Host unknown: ", $a, "\n";
                        #  ...
                }

                my $b = $line if $line =~ /User unknown/i && $line =~/^550/;
                if ( defined $b )  {
                        $b =~ s/550 5.1.1 (.+) \(.+/$1/;
                        $b =~ s/550 5.1.1 <(.+)>.+/$1/;
                        $b =~ s/550 <(.+)>.+/$1/;
                        print "User unknown 550: ", $b, "\n";
                        # ...
                }

                my  $c = $line if $line =~ /^<<< 554/;
                if (defined $c) {
                        $c =~ s/.+to (.+) cannot.+/$1/;
                        $c =~ s/.+account \((.+)\) .+/$1/;
                        print "User unknown 554: ", $c, "\n";
                        #..
                }       
       }
}

##</code><code>##

<<< 554 delivery error: dd Sorry your message to foo@yahoo.com cannot be delivered. This account has been disabled or discontinued [#102]. - mta210.mail.scd.yahoo.com