You must be logged in as the server’s root user in order to enable cPanel email relaying using Sendinblue. Also

1. Login on WHM Panel.

2. Now go to Home > Service Configuration > Exim Configuration Manager. And click on Advanced Editor.

3. Enter the below code  in the Section: AUTH Box.

Note: Use the username and apikey provided by Sendinblue when you verified your domain on Sendinblue.  

4. Add a below code of route in the Section: ROUTERSTART Box.

5. Add a transport to the Section: TRANSPORTSTART Configue Box:

6. Click on Save. Exim will restart automatically and the changes will take effect as soon as they are saved.