I rewrite RCPT in the Rewrite section of exim.conf using flag T.
I would like to store the original RCPT value before the Rewrite wipes
it out.
The manual states the RCPT can not be changed in Transports which is the
other place headers can be rewritten.
The earliest one can add a header is in ACL MAIL (using warn add_header)
but at that stage $recipients has been changed and $local_part and
$domain are empty.
