Our users are not getting activity notifications from CBOX. WordPress is able to send e-mail; new account invitations go out, password changes, confirmation links, etc., all work, and I see the outgoing e-mail in <kbd>/var/log/maillog</kbd>.
I set up two test accounts, one inside and one outside our domain (based on this post), subscribed to a group, and set them each for constant activity notifications. I then posted a new topic in that group, and replied to it a day later. In neither case is e-mail sent, nor is there any activity in <kbd>/var/log/maillog</kbd>.
Since WordPress is sending e-mail OK, it’s something about the CBOX suite of plugins, but I’m not really sure where to start looking. Pointers to configuration or code to start debugging this would be most welcome.