Is there a good way on how to become a back-up server for someone. The way 
listed in the postfix.org readme doesn't work:
put into main.cf
 relay_domains = the.backed-up.domain.tld
   relay_recipient_maps = hash:/ ..../postfix/relay_recipients
/....... /postfix/relay_recipients:
     @the.backed-up.domain.tld x
-- 
Thomas