locked Re: about messages from non-members
Here's a possible hitch I've come up with. If you are not a Groups.ioExcept in the case of your address being spoofed, I think it would be fair to conclude that your message itself constitutes opting-in for replies to that message. Otherwise what was the point?
Messages that pass SPF and/or DKIM I think you can accept as non-spoofed. For the rest, you may be right to immediately return an email confirmation message.
On a related note:
"Make the subscribe, unsubscribe, and other email commands reliable"