Is there a particular reason you don't want to import the accounts onto the linux system? I could see several problems with authenticating against POP3 on a remote machine:
1. Too much overhead. You're going across one or more networks just to authenticate a user when the process could be done instantantly on the linux system.
2. Lack of availability. What if the POP3 system goes down or some other intermediary system is unavailable. Your users should still be able to login to the linux system regardless of the POP3 system status.
3. POP3 is plaintext. User names and passwords would be sent in the clear across the network(s). To work around this, you could use SSH tunnels, or send data with some other form of encryption, but that just increases the overhead.
So why not import the accounts?