fold2 finding 4 (quality, Low): _verify_sender_authentication defaulted
authserv_id to "" (a silent fail-closed for any caller that forgot it) and
re-stripped/lowercased a value EmailAdapter.__init__ already normalises. Make it
a required keyword, test it with `if not authserv_id:`, and compare directly.
All callers (the adapter and the test helper) already pass normalised ids by
keyword, so no call site changes.
c495ab684b