Hi @Martin Cockshott ,
Welcome to our forum!
Based on my test on New Outlook for Windows: copy the To/CC field of a sent/received email and paste to the email body directly, i get the same result as yours.
As a workaround, it's suggested that you could open the sent/received email and click Forward, the To/CC field will be displayed as plain text. (The Forward message can be discarded)
Then you can copy the field you need. (The Forward message can be discarded).
Hope to help you!
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