[Exim] Strange rejects

What other options in 3.16 should cause:

2000-10-20 10:36:24 recipients from [10.5.50.84] refused (failed to find
host
name from IP address)

for local hosts, if host_lookup_nets is commented out ?

Exim has never been able to do reverse lookups on our internal hosts, but it
seems to have broken.

The changes I made which seems to have caused this is to add:

# host_reject_recipients = "*.exactis.com:*.easynet.net"
# recipients_reject_except = postmaster

and commenting these out again seems to have stopped the rejects, but leaves
me with a different problem.