Due to a quirk in the way Mailman handles undeliverable messages, you may find that you're occasionally notified that a large number of undeliverable addresses have been unsubscribed at once.
For example, you may find that you don't get any such notices for months on end, but then suddenly receive a notice that many of your list members have been automatically unsubscribed because they have undeliverable addresses.
This makes some people worry that there is something wrong with the system. However, this is actually normal with Mailman in some cases.
How Mailman removes "bouncing" addresses
Mailman removes subscribers when a certain number of messages "bounce" — but it resets the "bounce" tracking to zero if more than a certain number of days pass between bounces.
In other words, Mailman doesn't immediately unsubscribe list members because of a single bounce. It only removes them when multiple messages bounce, on separate days, within a certain period.
This is a smart way of handling it: it makes sure that people aren't removed if a single message is blocked by the recipient's spam filter, for example. It also makes sure that people aren't removed if their mailbox was just too full for a day.
However, it can mean that if there is always a gap of more than a few days between messages sent to the list, nobody will be unsubscribed, because the gap prevents the "several messages bounced within a certain period" rule from working.
When you finally do send several list messages in a row without a gap, all the addresses that have been "bouncing" for a long time will be unsubscribed at once. Most of these addresses have been "undeliverable" for ages, but there haven't previously been enough bounces for Mailman to decide it's safe to unsubscribe them before then.
How to make addresses be unsubscribed more quickly
If you do have a low-volume list that often goes a week or more between messages, you may want to increase the delay that Mailman waits before resetting the bounce tracking. To do that:
- Login to your Mailman list administration page.
- Click "Bounce processing" and locate the option marked "The number of days after which a member's bounce information is discarded, if no new bounces have been received in the interim".
- Change that from 7 days to something higher, such as 90 days. That way, bounces will be remembered for much longer, and bouncing addresses will be unsubscribed more quickly.