sub domain and mail exchange

[user]

I;m creating a sub domiain here and was curious if its possible for the sub
domain to have its own mail exchange? I've already created a sub.foo.com but
when I create a new MX record in the ns for sub.foo.com and the machine is
running sendmail ok. I can send and rec mail locally on the machine but when
I mail to user at sub.foo.com it gets bounced back saying server not found. Do
I need some sort of enry in the ns for foo.com as well?