Help! I just got a million bounces on my Mailman list!?!

This experience is a common one for new mailman lists. The scenario goes something like this:

  • A new list is created;
  • A whole bunch of email addresses are automatically added, either from a previous list, or from a sign up sheet;
  • Everything goes great for the first several posts;
  • Suddently the moderator of the list gets a large number of messages saying that member xyz has been unsubscribed for excessive bounces. The number of these messages can be anywhere from 10% to 60% of the people initially subscribed to the list.

The reasons this scenario happens is because Mailman keeps track of bounced messages. Every time a message is sent to your list, Mailman keeps track of the recipients that bounced the message back because the recipient doesn’t exist, or the recipient’s email is over quota, or the recipient is sending a vacation message, etc. Once Mailman receives about 5 bounces over a period of a single week (Mailman’s method is actually much more complex – this is an approximation), Mailman will automatically disable the account.

If, in the “Bounce Processing” section of your configuration, you have “notify owner on disable” then you should get an email every time someone’s account has been disabled.

Once disabled, Mailman sends a series of “probes” – test emails to see if it continues to bounce. After a certain number of bounced probes, Mailman will unsubscribe the email (permanently remove it from the subscriber list).

If you have “notify owner on unsubscribe” you will be notified when this happens as well.

Often you get a large number of these bounce notifications at the same time. This experience happens because, when you mass subscribed people to the list, a good percentage of the email addresses were invalid to begin with (maybe a typo, or the address is no longer in use, etc.). Each of these bad addresses consistently bounce, so they all reach the magic threshold of being unsubscribed at the same time.

We realize this is a very disconcerting experience! But, if we didn’t consistently unsubscribe bad addresses, our servers would be so over-whelmed sending and receiving bounce messages that they wouldn’t be able to send legit email.

If you are curious about who is getting disabled and why, you can log into the Mailman web interface. The disabled email addresses are still in your members list. However, they have the “nomail” box checked with a code in brackets (such as [B] for bounce). You can easily put them back on your list by unchecking the “nomail” box if you feel that Mailman disabled them when they should be receiving email.

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