Good idea. It is indeed how Sendmail (www.sendmail.org), QMail (www.qmail.org), Exim, etc. MTA's operate. Now implement it. You'll need a scheduler, etc., keeping track of the mailbox where the bounces arrive, matching them up with a database of messages that haven't been successfully handed off yet, etc. It's a big project, but absolutely essential for any big operation sending out a lot of email.