A common problem for some hosting managers nowadays is related to misconfigurations around IPv6, causing problems, for example, sending emails to Google.
CPANEL/WHM is no exception in some cases to this. But there is a way to disable (and it is simple) IPv6 from your EXIM server:
Select WHM Home –> Service Configuration –> Exim Configuration Manager –> Choose tab Advanced Editor
Scroll until you see the option “Add additional configuration setting”.
Select the following option from the drop-down menu:
Set the value to:
Scroll down to the end of the screen and click on:
Your server will now only use IPv4 to send emails and most likely all your issues around IPv6 will be mitigated.