Comment 1 for bug 265156

If bounce detection is turned on in Mailman 2.0, and the message was
detected as a bounce by one of the detector modules, then the loop would be
broken. Mailman 2.0 detects many more bounce formats than 1.x.

However if bounce detection were turned off, or the bouncing MTA had a
bizarre unrecognized format, then the loop can still occur.

As a fallback, Mailman 2.0 will look at the sender address and if it comes
from a list of well-known MTA generated addresses, the message will not be
forwarded on to the admins.