I sent a message to one of my groups and strangely received 2 "Delivery Failure¡± email notices
Perhaps these notices were from mail2world.com, not comcast.
Those messages were auto-forwarded to mail2world.com, some members of the .io group have set up forwarding. Probably you can see these members' email addresses in the header or footer included in the Delivery Failure notices. Unsub or ban them.
Lena