Postfix as domain MX20 (second mailserver)

[Holló Márta <hollomarta1@gmail.com>]

Dear List members,

I would like to configure my Postfix (MTA) - Dovecot IMAP/POP as a 
second mail server of my internet domain.
So, my MX20 should send and receive all mails to/from my ISP's first 
mail server (MX10).

How will I configure my /etc/postfix/main.cf , if my  Postfix 
send/and/receive all message from/to my  ISP's mail server?

And how can I configure my fetchmail (if my fetchmail will download
all mails from MX10 IMAP/POP server)?

Can anybody help me?

Marta Hollo