Hi there, Gabriel!
Thanks for reaching out to the community!
IMAP is an extension in PHP and is required in order to process IMAP on your server via PHP execution. To install the PHP extension, follow the steps below.
Login into your WHM panel via URL https://server-IP-Address:2087.
In the left-hand menu pane, type EasyApache and you will see EasyApache4 option appearing.
Click on EasyApache4 option.
On the next screen, click on Customize button on current Apache settings. It will load your current Apache settings.
Click the PHP Extensions option on the left side.
Enable your choice of PHP modules for the PHP version you select (in this case you'll want to search for IMAP) and click on Next.
Once necessary PHP modules are selected, the setup will review all the selection. It may take between 2 to 15 minutes based on a number of selected modules.
On the next screen, the setup will show the list of packages to be installed.
Click on Provision button to install the packages you have selected.
Michael Kinder's post in The PHP IMAP extension is required was marked as the answer