[Mailman-Users] Mass defection?

[Julio A. Cartaya]

A customer of mine has a broadcast (i.e. one-way), low-traffic list with
close to 3000 members. The list contains a mix of business and personal
addresses.  As usual, the list of member addresses can be seen only by
administrators (private_roster).

The date of the last message broadcasted to members is July 17(this
year).  On Aug 7 we received 2700 messages stating "johndoe at somesite.com
has cancelled his subscription to mylist" (where johndoe at somesite.com
changes - of course - for each cancellation message).

Other relevant data

   * This list has been stable, hovering around 3000 members for 3 years
   * It is unlikely all 2700 decided to drop from the list at the same time
   * The host logs show no sign of intrusion
   * I am inclined to dismiss vandalism, since there are 300 members
     whose suscription was not affected
   * There was no obvious pattern on the email addresses that were dropped

Note this is happening in *Spain*, where the entire country closes
during July and August and flocks to the Mediterranean, so chances are
that many mailboxes are full and rejecting incoming messages. The July
17th message could have caused an avalanche of bounced messages, and
automated bounce processing may have dropped all of those addresses a
few days later (bounce_score_threshold is 4.0,
bounce_you_are_disabled_warnings is 3).

In the end, my questions are

  1. *Is bounce processing a plausible cause (all 2700 addresses at the
     same time)?*
  2. *I found no way to distinguish user-driven dropout messages, from
     dropout messages caused by bounce processing; is there any?
     *
  3. *Is there a way to avoid this from happening again next year?
     (other than setting bounce_processing to 'no')*

Clearly this is not the end of the World (we had backups, and members
have been invited to re-join the list), but it is very unsettling for
my customer.

Thanks

Julio
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