When a bounce is received, Mailman tries to extract two pieces of information from the message: the address of the member the message was intended for, and the severity of the problem causing the bounce. The severity can be either hard or soft meaning either a fatal error occurred, or a transient error occurred. When in doubt, a hard severity is used.

If no member address can be extracted from the bounce, then the bounce is usually discarded. Otherwise, each member is assigned a bounce score and every time we encounter a bounce from this member we increment the score. Hard bounces increment by 1 while soft bounces increment by 0.5. We only increment the bounce score once per day, so even if we receive ten hard bounces from a member per day, their score will increase by only 1 for that day.

When a member's bounce score is greater than the bounce score threshold, the subscription is disabled. Once disabled, the member will not receive any postings from the list until their membership is explicitly re-enabled (either by the list administrator or the user). However, they will receive occasional reminders that their membership has been disabled, and these reminders will include information about how to re-enable their membership.

You can control both the number of reminders the member will receive and the frequency with which these reminders are sent.

There is one other important configuration variable; after a certain period of time -- during which no bounces from the member are received -- the bounce information is considered stale and discarded. Thus by adjusting this value, and the score threshold, you can control how quickly bouncing members are disabled. You should tune both of these to the frequency and traffic volume of your list.