[exim] clarification of the bang (for NOT)

I am confused over the placement of the ! to exclude hosts

for a whitelist (to exempt the IPs in the host list)
I have:

#check for rdns - if no PTR record, deny
deny message = We do not accept messages from hosts without reverse DNS $sender_host_address
!hosts = /etc/popb4smtp : cdb;/etc/whitelist-rdns.cdb
log_message = No reverse DNS $sender_host_address
condition = ${if eq{$sender_host_name}{}{true}{false}

I am confused if the hosts should read instead:

hosts = !/etc/popb4smtp : !cdb;/etc/whitelist-rnd.cdb

The reason I ask is that I had rejections from IPs which were/are in the whitelist.
So I am unsure of the cdb read failed, or the syntax is incorrect in the hosts=

Thanks!