There actually is a standard format for bounces. RFC 3462[1] defines Multipart/Report, a multipart MIME type for sending back delivery reports. Most big-name mail providers send bounces in this format these days, but there are still zillions of other MTAs out there that send in their own peculiar variation on text/plain. For example, last week while churning through a pile of bounces while writing a bounce processor I came across one that declared it was text/plain but then included quoted-printable HTML of a report page and some of the original message. No headers, of course.

[1]: http://tools.ietf.org/html/rfc3462