check_to() and check_from() from uri_db are basically not documented.
All the documentation says is that the To username is "checked against the URI table". No word about what exactly is checked (maybe that no user by that name does exist?), and no specification as to what return values to expect for what outcome of the check. Also, what is said in the section about check_to() concerning use_uri_table seems to contradict the paragraph about use_uri_table itself.