I've got a very small list (five members) for the management of a charitable organization, and we're having trouble with intermittent message delivery failure. I was running all the mailman related packages at git master from maybe two months ago, and updated everything to master again when I saw the announcement about version 3.2.

Two of our subscribers intermittently don't receive emails sent to the list by the other. One is using Gmail, the other Yahoo, and they confirm that messages are not being shunted into spam folders. It happens only occasionally: apparently nearly half the messages don't go through.

The rest of us get all the messages, including all sent by these two. All messages are correctly archived in hyperkitty. smtp.log always shows "for 5 recips", even for the messages that the two users aren't getting. These two users only ever miss messages sent by the other.

I'm still inclined to blame Gmail and/or Yahoo, though I don't know why they would be swallowing only messages from the other, and not from other list members.

Is there any way to get a better sense of where things are going wrong? Turn up the logging verbosity? Send test messages? Get more diagnostics?

If anyone has any suggestions (or if this situation sounds familiar to anyone) I would love to hear about it.

Thanks, Eric