I believe I have found the culprit in my particular situation.? I had a user who was getting the error "510: The group 'username=yahoo.com' does not exist"? every time she tried to reply to a group post.? This is what I found...
She uses a Samsung Android phone with the Samsung Email App.? It appears that this app does not use the "Reply-To: " field in the email header but uses the "From: " field instead when replying to a group post.?
Furthermore, If the message was sent from someone with a Yahoo email account, the "From: " field contains the funny "On behalf of" address (e.g. "username =yahoo.com at groups.io ").? If the message was sent from someone with a Gmail account, the "From: " field contains?the originator's Gmail address.??So, in the case of replying to a Yahoo sent message the user gets the "510: The group 'username=yahoo.com' does not exist".? In the case of a Gmail sent message, the reply goes directly to the originator, but totally bypasses groups.io.
I instructed my user on how to use the reply links which are within the message.? She is also going to switch to the Yahoo Email App, which seems to correctly grab the "Reply-To:" address.? Perhaps others who have Samsung phones can test this to confirm.? Also, the iphone Samsung app would need to be checked, but I don't have one of those.
Jim Horne