[exim] Header_syntax question.

HI, all

    I have this in my acl to veify syntax:

        require verify        = header_syntax
        require verify        = header_sender
        accept  senders       = !:
        require verify        = header_sender

and works great, but the only problem is that if I receive a mail
from: <> is rejected (administrative prohibition). I removed the line

  "accept  senders       = !: "

but the problem persists. Is there anyway to integrate both? Check the
syntax and permit the mail from null ?. Thanks

--
Guillermo Llenas