SETUP: I check multiple gmail accounts from Thunderbird via IMAP: bills@gmail.moc personal@gmail.moc business@gmail.moc I also have a couple forwarding setups: contact@mybusiness.moc -> business@gmail.moc (this domain is managed by google and google does the forwarding) oldaddress@gmail.moc -> bills@gmail.moc (I set this to forward through the gmail web interface; I don't check this old address directly) I try to keep my setup simple. I don't do any folder sorting; I just keep messages in "inbox" until I archive it. I don't have any plugins. I don't have any identities other than what is made with each account. I am updated to the latest TB version, 102.12.0 (64-bit). PROBLEM: #1: I send an email from personal@gmail.moc to contact@mybusiness.moc. The message eventually arrives in the inbox of business@gmail.moc. When I reply to this message (reply or reply all), a compose window opens with: From = personal@gmail.moc To = contact@mybusiness.moc These should be: From = business@gmail.moc To = personal@gmail.moc I get the expected behavior when: I send from someone else's account external@gmail.moc to my contact@mybusiness.moc. I send from personal@gmail.moc to business@gmail.moc. (This result is important.) #2: I get a similar problem when I send an email from personal@gmail.moc to oldaddress@gmail.moc. I eventually get the message at bills@gmail.moc. When I click reply, the compose window shows: From = personal@gmail.moc To = oldaddress@gmail.moc These should be: From = bills@gmail.moc To = personal@gmail.moc DISCUSSION: It seems that when a message is forwarded to another address AND Thunderbird has BOTH the originator account and the recipient account, a reply to that message picks up the wrong FROM and TO fields. When testing this setup, I initially thought the problem was more widespread affecting external accounts too but now I see that it is only if the same instance of TB has both accounts. I suppose as it is, this isn't a big problem as I don't actually send messages to myself. I only did this for testing and it confused me for a while. I have seen a couple other posts mention this problem. The suggested solution is to configure identities but this /has/ to be a bug that should be resolved. It doesn't make sense that it would work like this. RELATED: Errors populating To, From, and Reply-To fields on replies https://support.mozilla.org/en-US/questions/1239772 How can I reply to the sender instead of replying to my alias I got the email from? https://support.mozilla.org/en-US/questions/1370144