Thanks for your reply.
Basically i have an IaaS (Infrastructure as a Service) where i can make VM, VPN, etc.
IaaS managing for me DDOS, mirroring servers, back-up space, etc. (at a big lvl)
On that console i set the network adapter for in/out from public IP to private IP in any direction with any port, so not using the NAT ip from CentOS WebPanel.
rDNS is set for the main domain and IP of the server.
I also set Dmark, SPF, DKIM, based on the several online tools, my email server is set correctly.
I can send emails for any other domains that are outside my VM (gmail, yahoo, ymail, etc) and everything is working like a charm.
I ran into a similar problem when i try to curl from VM1 to VM2, i added in /etc/hosts the domain with the local ip and curl work perfectly. I'm wondering if i have to do something similar.
Also 25, 465, 587 are open both direction, do i need to add in some conf file the same option like i did in /etc/hosts?