responding how?

Thunderbird automatically sends a "Reply" email to every email I receive. I know because the replies to "no reply" emails bounce into my "Sent" box.

I have no idea, it can not be a delivery notification as a no reply email would not be requesting one of those. So I am left with asking you to identify what the bounced email actually says, perhaps it something from a filter you have created is all I can think of. Knowing the content of the message that bounced might offer some insight as to the source.

Do you have an automatic response set up at the server (through web mail)?