The main problem is you are looking for an answer in the wrong place. The PHP code is not the better way to look for errors in sending messages with a mailing list. That's the work of the own mailing list program. I don't know what type of mailing list program you are using, but Majordomo, Mailing List Manager, ListServ and several more programs can help you greatly with this problem. All of them can bounce the wrong messages to the desire email address. Just check the manual of those programs to know how (sorry, the answer to this is sometimes very long).