[Exim] virtual domain

Hi list!

I set my exim to attend a virtual host.
I don't understand hows it works. I send a letter to
username@??? the error message is : 550 Unknown local part
username in <username@???> .
The next is how 'll the recipient address is @virtual.domain,
not @original.domain. I have red Exim Virtual host FAQ
and i didn't find any docs about it?
Can You offer me a good one, or help me please.

Best regards
Gergely Kodaj