[Exim] Being a secondary MX

We have a few domains that want to serve as a backup MX incase thier primary
mail server fails. I tried adding the domain to the list of local_domains, but
I get errors when exim cannot find an entry in aliases or the password file.

What is the best way to provide backup MX for clients?

-- 
    -Jason J. Horton <jason@???>
     Fat Man in a Little Coat
     Intercom Online Inc. 
     212.376.7440 ext 21 | http://www.intercom.com