The account pane in account settings has a 'reply to' entry to use when you prefer other than the sending address. Also, on that pane, you may have set other identities. If so, check the 'manage identities' button.

@david, Hi David, thanks for the quick reply! That doesn't seem to be it. The email address listed in the Default Identity is correct. Just for kicks, I copied that "correct" email address into the reply-to address pane. No change. I checked identities under "manage Identities", and there's only one listed, which is the same as the default address. Hmm...I wonder if it's a gmail setting that causing this, as it is going to a Google account. Very puzzling. My other email accounts don't have this behavior.

Gmail has that capability, forwarding to a different address.

Oh heck. it's not a different email address at all. It's actually the name of the identity I must have set for the account. The email behind the identity is correct. My bad. Thanks David. Peter

I was going to ask whether this is a Gmail account when I read your first post… :)

You appear to be saying that it was a problem with the way you had set up that identity in Thunderbird. If you may fix the problem changing that, fine, but there could be something else going on here, namely that Gmail can be configured to apparently let you use its outgoing server to send from other addresses, but it doesn't really do that, what it does is silently change the "From" address of the messages you send through it to the Gmail address associated with the SMTP server…

This is why, contrary to what is recommended, I configure my Gmail accounts to save a copy of sent messages, only I save them to a Sent folder in Local Folders instead of to the Sent folder used by Gmail on the server. That way I may check the message that Thunderbird sent to Gmail SMTP server against the message that Gmail did actually send, in case I want to see how was the message silently altered by Gmail…