Ok, a little more information that might help.

I created a similar test, using 3 "native" Gmail accounts, let's call them "original", "forwarded", and "recipient". The "original" and "recipient" accounts are set up in TBird. From the web interface for forwarded@gmail.com, I set up a forwarding rule such that all incoming messages would be sent to recipient@gmail.com.

From TBird, I then sent a test message from original@gmail.com to forwarded@gmail.com. Sure enough it arrives in the recipient@gmail.com inbox in TBird. But the reply behavior is basically the same as I described in the original post.

However, I did notice that the From: address in the reply message was my default account (in this example, original@gmail.com). There was a drop-down box where I could select other From: addresses, so this could be the expected behavior in TBird. I'd just always need to remember to go change that before sending, which is a little inconvenient. If I compose a new message from recipient@gmail.com, the From: address is recipient@gmail.com (although I am given the drop-box where I can change it if desired).

But the To address was still the interim forwarded@gmail.com, which I believe is still not correct. Hoping this new example might jog some ideas here...

And as before, I have the full message header if that is useful.

Then please put the headers on pastebin and post a link here.

My experiences with most of these alias type things is that they tend to be proprietary in nature like the Outlook ALias feature and do not translate out of the web interface well at all. From what I have read, these alternative addresses might be a little mote "standards compliant" than most. What I have read (I have no access to workspace) indicates that it is the delivery mechanism in gmail that is special, not the email address to email. working somewhat as an account agregator rather than an alias. So having the headers will probably disclose a lot.

My guess as I said is that this is essentially only an aggregation process so you will need to add the alternatives to the account in Thunderbird as identities to get the appropriate from details inserted automatically when you reply.

In account settings, the default identity is the first pane above server settings and the manage identities allows specific details for multiple identities that use that account to be added.

Each secondary and subsequent identity contains this field which essentially identifies the email address the identity applies to although the thing can function with a wildcard (*@example.com), so as the image indicates all mail to a specific domain gets that identity used for the reply. So if you have a business domain redirected to a personal account then the business signature etc can be used for all replies that use that domain.

Thanks for the reply Matt. Yeah, I understand that Google might be using proprietary techniques, and I guess there's not a lot I can do about that. I also understand that the Workspace alternate address example is difficult to reproduce as most people don't have access to a Workspace account. That's why I added the gmail forwarding example, thinking it might be easier to set up. I am willing to work with you (or whoever) to provide as much diagnostic as possible.

But I was just wondering if there might be something TBird can tweak, or a workaround such as you suggested.

Here is the header from the forwarding example: https://pastebin.com/LEiVDYs1 Here is the header from the alternate email example: https://pastebin.com/ts7HPBWL

I think the mechanism for forwarding is different than the alternate, as the headers are quite different.

I will also experiment with the secondary identities. That might just be the trick. I suppose I would need to add identities for every forwarded gmail address and alternate email pointed to that account?

Matt, that was the magic tweak I was looking for! Obviously something I wasn't aware of... thanks for the tip.

One last little thing... the secondary identity (identities) has the Email Address set to (e.g.) forwarded@gmail.com, but nothing else set except the Private Data line you show above checked so that "Reply from this identity when delivery headers match: forwarded@gmail.com" is set.

Now when I reply to a message forwarded into recipient@gmail.com, the To: header is correctly set to original@gmail.com. But the From: header looks like this:

forwarded@gmail.com recipient@gmail.com

where recipient@gmail.com is italicized and grayed out.

When the reply message shows up in the original@gmail.com inbox, the From: header shows recipent@gmail.com, not forwarded@gmail.com.

I could probably live with this outcome, but maybe you are aware of one more thing I could set so that forwarded@gmail.com comes all the way through as the sender.

And btw, the secondary identities fix handles both the forwarding and alternate email cases. Both still have the odd reply issue described above.