Mysterious Reply-To Header in Apple Mail

Solution 1:

So with a bit of help the problem was tracked down to an interaction between SIP (System Integrity Protection), terminal, and Apple Mail.

Here's what was going on:

• Due to SIP the Terminal app did not have access to Mail's defaults file: ~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist
• As a result, the defaults program was unable to read the defaults settings that Mail.app was actually using.
• Instead of reporting an error when trying to access the com.apple.mail.plist it reported that the "The domain/default pair of (com.apple.mail, UserHeaders) does not exist."

The big clue was that defaults read com.apple.mail returned nothing.

The fix was to allow the Terminal.app full disk access in System Preferences (in Security & Privacy > Privacy > Full Disk Access). Then you restart Terminal.app.

After that defaults works as expected and

defaults delete com.apple.mail User-Headers

solved the problem – since I didn't have any other User-Headers set.