For Email services
Exim is an open source mail transfer agent (MTA), which is a program responsible for receiving, routing, and delivering e-mail messages.
Exim can be run on any TCP/IP network, in conjunction with any combination of host and user software, and is the default MTA included on some Linux systems.
Exim port in VPS /Dedicated server
Many ISPs (Internet Service Providers) block port 25. It is a great advantage to have an alternate port that is available to use to avoid interruption in sending out emails.
For example, to set the port 26 (465 or 587) to be available for usage:
1.Log in to WHM.
2. Click on Service Configuration.
3. Click on Service Manager.
4.Under exim, enable the SMTP Server.
5.Under exim on another port, make sure the port is set to 26.
Thats it the exim port has been changed now.
And to know how to configure mail server on centos click here.