Subdomains and strange mail problem

We have Discourse running fine. It’s at domain Mails go out fine, and all users can send replies by email OK. Except just one user. The Reply address he is getting in his mailer is the domain not

I am utterly baffled as to what/who/where is causing this address rewriting. It’s a showstopper to our site for various reasons.

I am using Amazon SES but I am fairly confident the problem does not lie there, as all the other users get the right reply address.

Has anybody ever seen anything like this before?

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Try to find the differences or commonalities to narrow it down:

  1. Is there anyone else with the same email domain/service as this user, and do they have the same problem?
  2. What about the email client he’s using, and is anyone else using the same one?

Thanks. Nobody else with same service (luxsci). Client is up to date Thunderbird 68.6. I am very baffled. I spend a large amount of my time setting up enterprise Postfix systems, but I have never seem this type of error. There’s something new every day!

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Does he have any add-ons in Thunderbird that might be interfering here? A cursory search for thunderbird changing reply-to came up with this, for example:

Also, it might help if you can get the raw email that he receives and maybe having him try a different email client or webmail if available to test.