Very serious e-mail problems between Cpanel servers

Dear admin and advanced users of Cpanel,

I seriously need you help on this. I am a reseller of a hosting company and they uses several Cpanel servers. There is a serious e-mail problem in between different Cpanels that I reported months ago and is still not resolved by the hosting company.


(E-mail accounts in general)
The problem is that e-mail accounts setup on Server1 cannot be received at Server2, 3, 4.. etc. In other words, all e-mail accounts on the same server can communicate with each other, but any e-mail going to different servers are lost without any error message.

To put the story short, if I have 50 domain accounts setup on each servers, and each has 2 e-mail accounts. All 100 e-mail addresses' e-mails on Server 3 cannot be received at any e-mail account of other servers.

Please give any comment if you can. I seriously need a lot of help on this.

Thank you in advance!

 

I would like to add that the e-mail problem is only in between Cpanel server e-mail accounts. I could send and receive e-mails just fine with others like yahoo, hotmail, etc.

 

Thank you very much for your reply. Hopefully the data center is able to solve this problem.