HelenaGwyn the domain "1.srs.us.opalstack.email" is the domain used by our SRS setup for forwarded mail.
SRS changes the envelope sender to use an address that's covered by the SPF records for the forwarding domain, this prevents SPF failures which would otherwise cause the forwarded message to be blocked. The envelope sender is separate from the "From" header.
It looks like Gmail is replying to the envelope sender instead of the "From" address which feels like the wrong behavior to me - after all, when you click "Reply" to a forwarded message in Gmail, it goes to the correct address so I don't see why an automatic reply should be handled any differently.
That said, we'll look into this to see if there's anything we can tweak in our SRS setup to improve this.
In the interim you might be able to work around the problem by implementing an autoresponder on a self-emptying mailbox as described here.
