Ok, I'm having an email problem in which if clients use our main domain nameserver(s) DNS, they can send/recieve email fine both to third party services and between our local servers.
However, if my resellers use virtual DNS nameservers I have created for them, they can send to third party servers, but NOT to our local servers located at the same colo. HTTP web services using virtual DNS works fine, though.

Anyone have any idea what can be causing this? Am I making a mistake in setting up DNS?

Here is my basic DNS configuration:
Main Nameserver:
A record - ns1.nameserver1.com --> IP Address1
A record - ns2.nameserver1.com --> IP Address2
Reverse PTR IP Address1 ---> ns1.nameserver1.com
Reverse PTR IP Address2 ---> ns2.nameserver1.com

For virtual nameservers for reseller clients, I have just done basically the same above but have delegated a new IP.

Then, I have setup virtual sites DNS like this:
A record - domain.com ---> IP Address
A record - www.domain.com --> IP Address
Mail MX Record domain.com -High priority mail server --> www.domain.com
I have also entered the correct nameservers in the SOA settings.

Am I missing anything is setting up nameservers, other then creating A records for the nameserver IP(s) and reverse PTR records? I see under the "add" menu, "secondary name service for domain" and "secondary name service for network". Do I need to do something with this. I appreciate any help from network gurus:)