If you are using an EC2 instance with Amazon Web Services, you may run into issues sending email due to port 25 being blocked. While Amazon may life this restriction, it is possible that they will refuse to unblock port 25.
The first thing you want to do is contact them and ask for the restriction to be lifted using the information in the following article.
If Amazon will not allow port 25 to be opened on your EC2 instance, you can look into using Amazon's SES (Simple Email Service) for sending email.
You can also set up a Smarthost on your cPanel server that allows the routing of email through a remote SMTP server.
Beyond these options, you can look into using other third-party mail services for the relaying of email such as GMail. Reaching out to your web hosting provider for available email options is always recommended as well.