[exim] only relay mail for our domain in relay_from_hosts..

We had 2 accounts get compromised in the latest 'please reply with your
password ( ) ' scam..

so what I am looking to do to prevent this from impacting us in the
future is..

I would like exim to *only* send mail if it is from our domain..

in the acl_smtp_mail I have tried..

acl_check_mail:
require domains = mydomain.org
accept

which passes exim -bV but gives a 451 and: cannot test domains
condition in MAIL ACL


How would I accomplish what I am looking for..

Thanks in advance.