Well, let’s take imaginary case that you have forgotten your POP3 password.
Then how you could try to find it back?
Answer: Set up pop3 server locally in your computer, change server address into it in mail client, and cross your fingers…
Interesting… Comment writing doesn’t work anylonger.
Anyway much simpler approach is
(1) Take netcat
nc -l p 1025 localhost
(2) Connect to localhost:1025 with POP3 client
(3) Simulate POP3 server on netcat
That’s it!