They don't show in "sent items" under "to" and they don't show in the
body of the email when forwarded. In fact, the "To:" line disappears
altogether for that person.

This has only happened with one addressee (so far) and happens every
time an email is sent to them, even when typing the address each time.
The person is not listed as a contact so the problem isn't there.

Any ideas why?