(sorry about my English)

I have this records
www.domain.com ---> ip
domain.com ---> ip
mx domain.com ---> domain.com
sub.domain.com ---> ip

I want to have a email for subdomain
like xxxx@sub.domain.com
and it is work good....

Question: must put for safe a mx record for the sub.domain.com ?
mx sub.domain.com ---> sub.domain.com

thanks