[Exim] FROM address from sending host is invalid

What, EXACTLY does that message indicate?

There is a company who sends all their outbound mail thru a 3rd part
mail server (for virus scanning). That 3rd party then delivers it.
But when that happens, exim seems to be rejecting it with an error
message saying:

FROM address from sending host is invalid

How can I test the address from the command-line? I've tried

exim -bv <address>

and that checks out. So maybe I'm mis-understanding what that error
is trying to tell me?

Thanks for any pointers.

Jeremy
--
Jeremy D. Zawodny     |  Perl, Web, MySQL, Linux Magazine, Yahoo!
<Jeremy@???>  |  http://jeremy.zawodny.com/