MX question

[Jim]

hi folks,

I have a customer's dns server (bind - not sure of version) not able to 
resolve my mx records to one of my zones. Here are my current records:

 100 mydomain.com.s5a1.psmtp.com.
 200 mydomain.com.s5a2.psmtp.com.

I'm being told from the customer that my mx records are formatted 
improperly. His argument is that his version of bind is unable resolve the 
mx record when the domain name _does not_ appear at the end of the record 
(ie s5a1.psmtp.com.mydomain.com would work fine for him).

Is this is a known problem with some versions of bind?
Can someone point me to documentation that tells me proper/illegal syntax 
for my mx records.

Thanks,

Jim