Otherwise, looks good. I'm a little surprised we didn't explicitly handle
"Followup-To: poster", it's been around since RFC1036.
commit "nntpd: added 'newsaddheaders' option..."
The documentation doesn't describe what happens if the incoming message
already contains the To: or Reply-To: header fields. From reading the code, I
think they're passed through untouched, perhaps it would be nice to document
the intended semantics.