Not certain this is a bug rather than an unavoidable limitation. My expectations might be unrealistic :-)
If an email account is set up to auto-forward to a GIO group, then the resulting messages appear to be from the forwarding email mbox, not the original sender. Consequently, a "reply" to the post goes to the forwarding mbox and just loops back to the group (and I suspect that GIO is checking subscriber privs based on the forwarding mbox instead of the original source mbox). In the web interface, I see a From: header referring to the forwarding mbox, and the only reference to the actual originator is in a Return-Path: header.
This is different than when forwarding to an ordinary email address (at least a gmail address); then the From: header still refers to the original sender and a "reply" does the right thing. When the forwarding mbox is at yahoo, an additional X-Yahoo-Forwarded: From (forwarding mbox) To (final recipient) header is included (I don't know how forwarded-from-gmail messages look).
GIO's SMTP server or something must be re-writing headers in a way which defeats forwarding like this.
WHY DO I WANT THIS, you might ask? We formerly used an ordinary email mailbox for group messages; a "moderator" simply read the messages at that email account and forwarded them to the "subscribers". Now we are switching to GIO, and I want to make the old "posting email address" auto-forward to the group, so users will not have to learn a new posting address, and so existing user instructions will still be correct enough to be usable.
(Thanks for all the great work!)