exim configuration, mail for multiple domains

[Alex Dean]

I have Exim4 running on Debian Sarge.  I currently can only deliver mail 
to system accounts, and I'd like to figure out how to set up other email 
addresses which don't necessarily map to a system account.

For instance, right now the server is called 'example.com'.  Any mail 
sent to 'alex at example.com' is delivered to the local user 'alex'.  I 
have other websites on the server (Apache virtual hosts), like 
'other.com', but I don't know how to set up email accounts for 
'alex at other.com' or 'somebodyelse at other.com'.

I can't find any mention of how to do this online or in the Exim book I 
have.  Maybe I just don't know what the correct term to search for is?

thanks,
alex