[Exim] How to Secure SMTP/POP transaction

Hello,

We are running exim 3.16 on a RH server.
My question is : "How can SMTP/POP authentication process be secured
between my server and a remote client which is sending his password
in plaintext ?"

May the cram-md5 mechanism be the solution to my problem ? If yes,
any suggestion/idea of how to do that.

Any help would be appreciated. Thanks in advance.

--
Dat Phan