I’m having the same issue. I’m running 6.5.1, and using a mail only group “Mail” with only one forwarding recipient. Nothing has changed in my configuration since the last time this email worked other than DST patches.
If anyone has any guidance on this I’d certainly appreciate.
I had recently created a generic person with the last name Mail which was confusing my router as to which email address the email was suppose to be delivered to.
I got this problem when I used copy and paste to create a new group. The group had the same members but another Internet address. Deleting the document and creating a new fresh one solved it.