[Exim] Secure POP3

Hello,

When some users access the email server outside the local network, we can't
then put any restriction on the access to the server (i.e. prevent intuders
to use POP3).
Therefore is there a way to add security to the password authentification on
POP3?
For instance with an email client software, we can select the option to send
the password securely.
Is there any available software (freeware) for this under Linux to do this?

cheers
Guillaume