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 other techniques within cPanel server.
However, in some cases, customers may have "Virtual" email addresses, which are nothing more than forwarders and don’t actually exist within any server. This setting could cause problems.
Follow below steps to disable sender verification lookups.
- Log into WHM with 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.
Disabling this feature, may result in receiving more spam within your cPanel server. However, this will resolve any issues that you may have had with emails being unable to be delivered. There may be other techniques to combat spam might be employed on your cPanel VPS server such as SpamAssassin or RBLs (Real-Time Block Lists).