Help forwarding emails to another provider

Hi, I am newb on Linode and email administration in general. I have a new Linode box with CentOS installed and I have installed a web server with a few domains (for my own usage).

I have another 3rd party hosting provider "P" (reseller account) and I would like to keep P as my mail server. For example: I have 2 domains, domain1.com and domain2.com hosted in Lighttpd in my new Linode VPS. But, I want to keep info@domain1.com and info@domain2.com in my other provider.

I am a little confused about how to face this situation. I guess I need postfix and Dovecot but I need some tips how to forward or relay mail to my provider P.

Do you have any clue or hint about what steps to follow? or any recommendation?

I think I need to add entries to transport map table in postfix for sending emails, but probably I need to do something with Dovecot to receive external emails too. Or, maybe I can configure the DNS and MX records to jump above the server?

Thank you very much.