Problem with ccTLD's DNS resolution

Hello there. I'm having a hard time for at least a week trying to figure out a problem I have (I guess). It all started because I noticed that some customers were able to send mails to Gmail but couldn't receive. I searched in exim_logs with no success and 24 hour later I started to get all bounce backs: DNS Error: 463875 DNS type 'mx' lookup of domain.tld responded with code SERVFAIL
It was strange since I can see in WHM's zone editor everything is ok for that domain name. I then realized that this problem only happens with customers who owns ccTLD domains. I have done some testing with Top TLD's and it works perfectly, even when the DNS zone for both of them are identical. Here's an example of what I'm talking about. Example 1 - I own this domain and is currently hosted in my VPS: [removed URL] Example 2 - I own this domain and is also currently in my VPS: [removed URL] Also, here goes some testing I did inside my VPS: [removed due to the use of real domain names and IP addresses] After using the search function I found that some people had a problem where their Child Name Server weren't registered at the domain registrar. In my case, it's, and it's perfectly pointing to my VPS's ip address. (Hence it resolves for TLD customers) but for some stranger thing it doesn't for ccTLD and that's why my customers can't receive emails from Gmail. Also, I can't ping/whois ns1.removed.tld, neither ns2..removed.tld (from my side) but at the same time I can't understand how are they actually working for TLD domains? Any help would be appreciate.