Received from Shal Farley at 8/4/2018 06:06 PM UTC:
Toby,Or add a footer to the message showing the address itI like that choice.
So do I.
What I wouldn't like is for Groups.io to reveal the Owner's personal email address to someone sending email to GROUP+owner.
A slight catch to this approach is that the Owner/Recipient who has forgotten which email address he used when creating his group has to check mail at all the addresses he owns until he finds this email.