Over on the Group Manager's Forum, it was suggested that I raise this problem here. Please read through the topic with the same name as this one on that group to see the full story, but in essence the + in the email commands is causing difficulty because some routes in the internet are stripping-out anything after a + in the local part of an email address. This results in a user who sends an email command finding it appears on the group broadcast, (and the command is not executed). The effect can be embarrassing in the +owner command, where a private message becomes public.

This practice by some intermediate hosts in the internet may well be "illegal" according to RFC 2821 but my intuition tells me it won't go away,so I would ask groups.io to consider changing the format of the email command. This can be done step-by-step, adding the new form alongside the existing + form then later deleting the + form when it becomes clear that there are no users still using it. I suggest something like <mygroups.command@groups.io> - the use of a dot in the local part of an email address is a well-established practice.

regards
Peter