[Exim] Virtual domains?

Hello,

how can I set up virtual domains when I have data like this:

localpart@??? -> some.user@???

That means a customer has a domain and wants mails to any arbitrary
localpart at his domain to be forwarded to another arbitrary address. By
the way, these mails are frozen and exim is not the wanted mailserver,
only the frozen mails have to be sent.

I really don't have any idea about exim, not even if it is suitable to
use virtual domains for that purpose. I found something about virtual
domains, but these mails are send to specific hosts explicitely.

Thank you
Marco