Currently reading up on the IMAP protocol, as one of my customers is using a server without IMAP support compiled :)

RFC 3501

I will be using a simple INBOX check for failed email notifications (sockets are your best friend). That will allow me  to set a flag for every new user account that is still unconfirmed and used a broken email address for their registration. These accounts can than be verified manually :)

Happy socket = happy customer.

Btw, here is a nice piece of code to quickly parse the email header in PHP