POP mail server will not accept passwords

I am having a problem with my pop mail server. Uses can log into the OSX 10.3.5 server without any problems but when I try and access the pop mail server using the mail client on the server it says it does not recognise the password. I can telnet into the pop server and it accepts the username but not the password. This is the same for all users on the server. I know my password is correct because I can log into the server from a workstation.