[CentOS][Webmin] How to set up POP3 and SMTP?

Hi.
I have a VPS running CentOS 6 x64 and Webmin CP.
I configured Postfix Mail Server and I can send and receive emails in Webmin, but I couldn't set up POP3.

I tried to configure Dovecot IMAP/POP3 Server, but when I try to connect to the POP3, it says user or pass is incorrect.

I'm trying to send a message via SMTP, but I'm getting this message:
The message could not be sent because the server rejected the sender's email address.
I think I have to add a sender's email address in the postfix, but how can I do it?

I couldn't find any useful tutorial.

How can I set them up?

Thanks.