In your /var/named/domain.com.db you need to have a record like this if you use your own mail server:
domain.com. IN MX 0 mail.domain.com.
If you use Google for your emails you need to put Google records. If you use Zoho it looks like this:
@ IN MX 10 mx.zoho.com.
@ IN MX 20 mx2.zoho.com.
@ IN MX 50 mx3.zoho.com.