First, I don't know what your exact problem is, but I duplicated your situation and it worked as anticipated: - I sent from a legit email address (address A) to an alias of address B - the test message appeared in inbox of address B as it should - i clicked reply and the from showed address B (as it should) and the recipient as address A (as it should).

So, there is something else involved in your issue. Use of aliases is fairly common, yet your problem I've never seen before.

Thanks for trying it out. Let me try and make a video showcasing it so it may be better understood (in case that is a problem).

Just to clarify: Both A & B are setup in Thunderbird right?

I should've mentioned this initially but I am on `115.10.1-1`

I think I will chime in that most of these Alias things only work if you use the web mail provided by the provider, not a mail client. This is especially so for the outlook implementation. So I always have qualms when I know nothing of the provider. This is wild west country.

However Thunderbird's identity feature sounds like it is not being used here, but your example with different domains leave me struggling.

in your email source Return-Path: <poiasdpoiasd@cock.li> is the From: when you click reply, that is per standard. You set the return paths in the identity section for each identity that uses that incoming account, you also set the SMTP server to be used for that identity.

I mention the per identity because while Thunderbird uses identity information, like I mentioned for outlook, not all providers honor what is set and rewrite it to what is recorded on their web site for that identity. So if the primary identity you are using is BOB with a secondary configured in Thunderbird as "Old Bob" then outlook at least will rewrite that to BOB because that is what they have on their web site. There are further complications where Gmail SMTP servers are used, but I think that might be off topic for this question.

I think what we need is the source of the message from the sent folder and from the incoming folder. This forum handles such things with the finesse of a sledge hammer, so could you post them to the Mozilla paste bin. https://pastebin.mozilla.org/ One paste per message, and set the retention to 21 days after you paste. I doubt I will see your message for a day most of the time.

Remember to click the paste snippet button after you paste your source and then post the links to the snippets here.

Thanks for the reply @Matt. To simplify things, I've decided to just use my two "production" email (live.com and cock.li) instead of my self-hosted E-Mail.

I have sent an email FROM live.com TO cock.li, one with alias, and one without. Both E-Mails were sent using outlook.com in the browser (aka "webmail").

Here is a video of the reply behavior in Thunderbird when ONLY cock.li is setup: https://streamable.com/5nkjah . As you can see, reply in both cases works as expected.

Then, I just added my `live.com` E-Mail to Thunderbird as well. Here is the video with BOTH setup: https://streamable.com/9j4193 . As you can see, now the reply on the Aliased E-Mail is broken.

Here are the message headers for both scenarios:

• WITHOUT ALIAS: live.com headers from "sent": https://pastebin.mozilla.org/OQuOaoxZ
• WITHOUT ALIAS: cock.li headers from the received email: https://pastebin.mozilla.org/2ZuspLjy

• WITH ALIAS: live.com headers from "sent": https://pastebin.mozilla.org/DfYu7HHe
• WITH ALIAS: cock.li headers from the received email: https://pastebin.mozilla.org/fHoRuhiT