[Exim] Relaying prohibited by administrator

My customers IP's are in the subnet of 204.214.110.128/27, and also
206.163.169.32/27. I want people with IP addresses, and these IP
addresses only to be able to relay through my server. I tried using
sender_net_accept_relay, but seems that it doesn't work with 3.02. Can't
really go by hostname or revers IP, because those can be faked easily. It
is harder to spoof an IP, especailly when one's router is setup with
access-lists correctly.

How do I make it so that only MY dialup customers can relay mail off of my
mail server?

Their email addresses are @linkcc.com, but their IP is reversing as
*.crescentlink.com. This is how the company wanted me to set it up,
because people tend to spell crescentlink wrong, whereas linkcc they can
spell. So, mail server is mail.linkcc.com, but every other server, and
reverse Ip on the network is crescentlink.com. Just a little information,
in case you needed it...

Sincerely,

Richard G. Duvall