Why wont this MX record work properly?

[TIM MOORE]

Having some trouble with an MX entry.  Is this not a valid way to do MX
records (in the domain.us zone file)?
 
arete.domain.us   in  mx  10  mul8.domain.us
mul8.domain.us   in  a  xx.xx.xx.xx
 
 
If we send emails to user at arete.domain.us from certain mail servers
(Yahoo and Hotmail online accts) it works.  If we send from other email
accounts it doesnt work.  There is no actual zone file for
arete.domain.us, it is just a "domain" to send email to.
 
Suggestions?
 
Tim M.