cPanel sender verification lookups are built to ensure that the email address that the message is arriving from is a real and valid email address. If the email address doesn’t exist, then the message is rejected by the cPanel server. This is one method used to combat spam, as well as many other techniques within cPanel.
However, in some cases, customers may have "Virtual" email addresses, which are nothing more than forwarders and don’t actually exist within any server be it cPanel, Plesk or another type of control panel. This setting could cause problems.
To disable sender verification lookups, please do the following:
- Log into WHM on your cPanel server with your root password.
- Click Service Configuration, followed by Exim Configuration Editor.
- Uncheck the following two checkboxes under Mail:
- Use callouts to verify senders... (the first entry).
- Verify the existence of email senders.
- Save the settings.
Please note, that disabling this feature, may cause you to receive more spam within your cPanel Virtual Private Server. However, this will resolve any issues that you may have had with emails being unable to be delivered. Additionally, please note, that other techniques to combat spam might be employed on your cPanel VPS server such as SpamAssassin or RBLs (Real-Time Block Lists).