What does Mailman's "digest" checkbox do?
Mailman's "Membership Management" screen shows the subscribers to your list and the various options set for each subscriber. One of these options is the "digest" checkbox.
If a subscriber has the "digest" box checked, that subscriber will not receive each message as it's sent to the list. Instead, the day's messages will be grouped together and sent as a single message.
If your list sends several messages a day, the digest option gives the subscriber a way to cut down on the total number of messages in his or her "in box", while still seeing everything sent to the list.
Why do members get one last digest message after turning off digests?
When subscribers are changed from "digest" to regular messages, they still get one last "digest" message after that. This usually happens when the next message is sent to the list.
That's because it's possible that there were pending digest messages waiting to be sent to the subscriber at the moment he or she was switched to regular mode. If Mailman didn't send one final digest message to such subscribers, they would never receive any copy of the pending messages.
This only happens once. After that final digest message, the subscriber will receive no more of them.