[exim] Another questions for null sender

Hi

I'm using an exim server like a mail-relay to connect internal
mail-server to other company mail-server and external mail-server.
I'm trying to accept mail with null sender from an internal mail-server
to only another company mail-server.
For inbound mails I've insert the follow line:

acl_check_rcpt:

accept hosts = lsearch;/etc/exim/whitelist.cfg

But I've not idea to make for the outbound mails.

Some suggestion?

Andrea