(line wrapped, it's one line really, and it's a received_headers_text
entry, so modify accordingly). ASMTP_HEADER_SECRET is a macro with
some random foobar in it.
that makes you find any sender easily (as the "key space" is finite
and unlikely to be very big, unless you have several trilliongazillion
users :), but makes for any outsider hard to attack on your usernames.