[exim] hard coded alias entry to lookup

Hi folks.

I've got lots of the following /etc/aliases

user1:    :fail: This user address is no longer available. For assistance please 
contact paula@???
user2:    :fail: This user address is no longer available. For assistance please 
contact graham@???

This is long winded and also making my aliases file messy. How can I change
this to a rule, say in check_rcpt so that a separate file could simply hold

user1:    paula
user2:    graham

and use a lookup to generate the appropriate message?

Gary