Comment 4 for bug 265657

Norbert Bollow gives this very good idea:

Rate limiting can do a lot to prevent things from getting
out of hand. For example, respond at most ten times to
essentially the same request from the same email address.

On any given day, when Mailman gets the tenth message from the
same email address without valid commands, it could reply "This
is the tenth message of this type today from you. It order to
avoid problems like mail loops between email robots, any further
messages of this type will be ignored today. Please try again
tomorrow." And then any further messages from that address with
no valid commands will be just discarded.

Similarly, Mailman would reply only to ten subscription requests
for the same list from the same user on any given day. And only
to ten unsubscription requests for the same list from the same
user on any given day. And only to ten requests to change the
subscription options in the same way. Etc.

Since most loops (with the exception of some bounce loops)
iterate more quickly than ten times per day, this will kill most
loops between robots before they create serious problems.