I know this is very frustrating but lets pause a moment and see the possible reasons this could be happening.

1. 1.   Mail Server (SMTP) is Offline

This is a common issue to WHM/cPanel and Plesk servers.  Simply restarting the service from the control panel often fixes the issue.

How To Restart Services In WHM cPanel      

As an administrator with cpanel credentials you can restart services in the cpanel.

Steps

1. Login into cPanel with your root/administrator account.
2. Search for “Restart” using the search tool on the side panel
3.  The Services that can be restarted through cPanel are listed down.
4. Click on the service that you want to restart. For this example, we shall restart the SMTP service. A popup message prompting for an action will appear, when you click on the service. Click “Yes” to proceed.
5. Service will be restarted.
6. A success message will appear if the service is restarted successfully.

2.   Blocking of Port 25 by the ISP

Many ISPs force you to use their own SMTP servers even if you have your own.

You can test if the port is blocked by running the following command on CMD

telnet [domainname or ip] [port]

3.   SMTP Authentication

Some email systems use an older authentication method called POP before SMTP.  With this method, you only need to configure your IMAP or POP3 settings correctly. This likely means SMTP authentication is not enabled.