[Exim] Mail Masquerade

Hello,

Is there a way to masquerade mail addresses?

I have my own domain name. I want mail sent to that
domain name to end up on my ISP's POP3 server.

With mail sent to nathan@???, the To:
block needs to be rewritten to be nathan@my_isp.com.

Can this be easily done with aliases somehow?

Thanks,
Nathan