Prevent Thunderbird from stripping custom headers (e.g. X-Alias) on sending

Hi everyone, I'm working on a privacy-friendly email setup and ran into a challenge with Thunderbird. My setup: I have a catch-all email address that forwards all incoming emails sent to *@mydomain.com to my real mailbox, secret@mydomain.com. This way, I can use aliases like alias1@mydomain.com, alias2@mydomain.com, etc., without exposing my actual address. This part works perfectly. The problem: When replying to a message sent to alias1@mydomain.com, Thunderbird automatically uses secret@mydomain.com as the sender address. That defeats the purpose — the recipient now sees my secret address, which I wanted to keep hidden. Thunderbird offers a workaround via identities, but it requires creating a separate identity for every single alias, which becomes unmanageable if you use a lot of them. My attempted solution: My idea was to have my mail server insert a header like: X-Alias: alias1@mydomain.com Then, on sending, the server could rewrite the From: header using that value — thus preserving the alias and protecting my actual address. However, I discovered that Thunderbird strips custom headers like X-Alias when sending. That breaks the solution. My questions: 1. Is there any way to configure Thunderbird to preserve specific custom headers like X-Alias during sending? 2. Alternatively, is there a better or recommended approach to achieving what I’m trying to do — replying from the alias address automatically, without exposing the real one, and without creating hundreds of identities? I’d really appreciate any ideas, insights, or suggestions. Thank you!