Every MTA used to have their own way of reporting bounces. These days, the big ones (at least by volume) seem to all obey RFC 3464[1], which defines a MIME type

Every MTA used to have their own way of reporting bounces. These days, the big ones (at least by volume) seem to all obey RFC 3464[1], which defines a MIME type and required and optional fields for delivery reports. Of course, the old MTAs are still around and still delivering their bounces in a myriad of different formats. The bounce processor I work on at work has about a dozen different variants on "The message could not be delivered. There is a copy of the original message/headers below this line". Those plus the RFC 3464-style messages cover roughly 95% of what we see day to day. The other five percent are so unrecognizable that we can't process them automatically. [1]: http://tools.ietf.org/html/rfc3464 http://tools.ietf.org/html/rfc3464