/etc/dovecot/dovecot.conf

# Protocols we want to be serving.
protocols = imap pop3
disable_plaintext_auth=no
mail_location=mbox:~/mail:INBOX=/var/spool/mail/%u

listen=*,::

test:

telnet mail.iamtraining.com pop3

connected to localhost.
Escape character is '^]'.
+OK Dovecot ready.

user kaiming

+OK

pass password

+OK Logged in.