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 then I added my B account. Now all of my outgoing e-mail has the Reply-To header set to my A address. This causes considerable confusion to observant correspondents e-mailed through my B account. So, I'd like to fix this.
Here's where the mystery begins. I don't remember how I set the default Reply-To header. I would have thought that I would have used:
defaults write com.apple.mail UserHeaders ...
defaults read com.apple.mail UserHeaders
tells me "The domain/default pair of (com.apple.mail, UserHeaders) does not exist."
Next, I looked at the configuration on our e-mail provider, FastMail, to see if they might be setting a default Reply-To header. but I didn't see anything there.
Then, I tried resetting Apple Mail back to its default state using the instructions in How to reset mail to factory settings on discussions.apple.com. No dice.
And then, the method in How to Fix Mail App Not Working After macOS Update. Strike four.
Manually setting the header when I'm writing an e-mail doesn't work. The value I set gets overwritten, by the value for account A.
Where else could the header be set?