APOP, or POPAUTH, or whatever you want to call it, is a system whereby the password is encrypted using a "secret" that you give to the server beforehand. It's not true public-key encryption, but it's "good enough" for most purposes.
To enable it, log into the server and run the command "popauth". It'll ask you for a password - this *could* be your login password, but we recommend a separate password.
Fire up your email client; for the sake of argument we'll describe how to configure Eudora. Go to the "properties" settings for your POP account to this server. Under the "Incoming Mail" panel, towards the bottom you should see a choice of "Authentication Styles"; APOP is one of those options. Select it. Next time you connect, you'll be asked for a password - enter the same password you gave the "popauth" command. And voila, you should be in.
Fetchmail, the Unix-based POP mail retrival script, also supports APOP.
Of the free webmail services out their, the following have been tested and do not support APOP:
If anyone knows of a free webmail service that provides APOP support, let us know.