Why Do I Suddenly See A Large Amount Of Bounces On My Mailman List?

Sometimes there will be unexpected bursts of bounce messages on your mailing list. There are multiple reasons this might occur.

The underlying Mailman mailing list software has dedicated bounce handling processes. Temporary problems with disk space, memory usage, or the server's database service can cause these processes to fail. Eventually these processes are started, and any bounces that accumulated while the bounce handlers weren't running will be processed in one large batch - causing a temporary spike.

Some mail hosts (particularly Yahoo) will temporarily delay accepting messages rather than bouncing them immediately. However, these deferrals by the receiving mail host can eventually cause the message to time out in the server's queue and finally bounce. These bounces will often come in as a single large batch.

Other potential causes may include recent changes to your domain's SPF record (is your list server still authorized?) or the list server itself being added to a blocklist for their own arbitrary reasons.

If you have questions about such behavior, please contact your list provider's support so they can investigate and provide further details.