Yes, send me some log files. Actually the bounce detection should not be made reading the message content but using the return path, coded in the form
returnaddress+[code]@domain.tld
where the [code] represent a subscriber identification number. This is the only way to know who need to be marked as bounced, since the content of the error message is many time customized and some times the address reported is the final address of a chain and does not match the one used from the subscriber. I say that because I find all those cases.
Anyway, try to send me the logs. There is a library that should be used for that:
http://phpmailer.worxware.com/index.php?pg=bmh
And may be it could be used directly in the Newsletter plugin... I need to look at the license and see how to integrate it.