Sender uses wrong address - alias?

[isadora]

(Just as background. I was previously on the committee of the Ryerson Index, which was hosted by Ancestry.com.. Ancestry's mail servers were so unreliable tat the Ryerson moved to its own server.)
I have an account with Ancestry.com.au, using my email address <x@y.net.au. In Ancestry's Outbox, this has become "x@y.net. The result is that code groups sent to me for 2FA never arrive. I have reported my problem to Ancestry.com, but basically, they don't care. They referred me to their own documentation, which they are not following. My email client is Thunderbird.
So, I am having to find a workaround. There was a warning somewhere about editing the headers, and I am not sure whether the emails are arriving here. Any suggestions?

[doktor5000]

Please remove the full mail adresses, otherwise we'd have to remove them, personal information should not be posted publicly.

Apart from that, you cannot change the headers for mails that have not been sent yet. The sender identity needs to be changed on the sending server, or you need to setup an alias there. Nothing can be done about that on the client end, from what you described.