I recently created a Google Workspace account. There, I have a user profile with a custom email address. I've set up the new account in Thunderbird, and it is working as expected. If I send a message from my "external" account to the workspace account, it arrives, and I'm able to reply back. One feature of Google Workspace is what they call "alternate email" addresses. They sort of work as an alias for the native address. I have created one such alternate. If I send a message from my external account to the alias, it arrives correctly in the workspace account. However, when I reply from the workspace account, it does not go back to the originating address, but instead it is addressed back to the alias. If I repeat the experiment from the Gmail web interface, it replies to the correct address. I've downloaded all the headers, and the Return-Path tag is correct. I can provide the full header if needed. It feels like there is an invisible forwarding action to get from the alias to the workspace account, and that the reply action is incorrectly finding the interim alias. But that doesn't really seem to be what I see in the headers - although I'm probably not the most expert reader of those. And it isn't necessarily a Thunderbird issue. I can see that there could be something going on with Google. But this is a good place to start... :-)