I don't know what good it would do a spammer to know the number of bounced e-mails.
Anyway, lalichan, what you're going to have to do is use PHP's POP3/IMAP
functions or one of the many PHP POP3 libraries to check the mailbox to which the messages are bounced back to. You'll have to do some text processing to figure out which ones are bounces. That you'll have to figure out on your own.