I believe that this will work but you should test it.
In /etc/exim.conf:
Look for the line "accept hosts = +relay_hosts"
(without the quotes, and note that the spacing may very well differ)
and right under it create another line:
accept hosts = 123.45.67.89 : 123.45.67.90 : 123:47:98:99
where the IP#s are the ones you want all email accepted from, and they should be separated by the ":" character.
Be sure to restart exim.
Jeff