This issue has cost me more than 10 hours of the last two years: https://apple.stackexchange.com/questions/402204/mysterious-reply-to-header-in-apple-mail - I'm SO GLAD to have finally resolved it and now once again have a reply-to that makes sense ;)
Mysterious Reply-To Header in Apple Mail

I send e-mail with Apple Mail from two different accounts - lets call them A and B. Years ago I set the Reply-To header as my A account address (I think to force use of my 'canonical' address), since

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