[AFRI-Discuss] Questions about how email addresses get dropped from the mailing list

Nick Ashton-Hart Nick.Ashton-Hart at icann.org
Wed Jul 16 08:40:45 EDT 2008

Dear All:

Since every so often an email address gets dropped from the mailing list, and this has resulted in questions being asked about how this happens, we provide below a bit of an explanation.

The mailing list software in use by At-Large is called 'MailMan'. It is very widely used and is also widely known as a very reliable mailing list manager.

When an email is sent to a subscriber of the lists and is returned as undeliverable (known as a 'bounce'), 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, which in our lists is set to 5, 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 - all At-Large lists are set to send three reminders - that their membership has been disabled, and these reminders will include information about how to re-enable their membership. The reminders are sent once every seven days.

In short, what this means is that a subscriber would have his or her postings disabled if at least five emails were returned as undeliverable within 7 days. At that point, the list software will stop sending list messages, and start sending reminders that there is a problem, and asking the subscriber to fix the problem - once a week for three weeks. Only after that time, if no response is received, would the subscriber's email be removed from the mailing list itself.

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. In At-Large, this is set to 7 days. This means that then the calculating of bounces starts back to zero.



Nick Ashton-Hart
Director for At-Large
Internet Corporation for Assigned Names and Numbers (ICANN)
Main Tel: +33 (450) 40 46 88
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