Normally the From address is used for replies. However, could you check the account settings in Thunderbird to see whether a "Reply To" address is set for that account:

Or, if you have multiple Yahoo accounts, perhaps the outgoing server settings got crossed up between your accounts?

Thanks, but I haven't set a "Reply To" and I have only one Yahoo account.

A couple other things that may be relevant?

(1) To the best of my memory, this is the first time that a reply has come back to my primary Yahoo address. (2) The replies to both of my emails (from Thunderbird and directly from Yahoo) were "auto."

Any other thoughts very welcome!

The most probable issue is yahoo simply replace the from address with the one they associate with the username and password used to send the mail. Hotmail/outlook have been doing it for years and years. Their alias feature simply does not work from any mail client. I guess the situation is the same with yahoo disposable addresses.

Thanks Matt, but the fact is that I've used Yahoo disposable addresses for years now without this hitch...and in fact continue to, except for this one single instance. Is it possible that this particular recipient can read information about me? Is it the "header" where this info might be stored?