To configure cPanel emails to relay via Sendinblue, ensure that you are the root user of the server.

Login to your cPanel & WHM Admin Panel as Administrator. Go to Home >> Service Configuration >> Exim Configuration Manager.

Click on Advanced Editor.

Enter the following in the AUTH Box (replace username and apikey with the actual values. Learn more here.):

sendinblue_login:
  driver = plaintext
  public_name = LOGIN
  client_send = : username : apikey

Add a route in the ROUTERSTART Configuration Box

send_via_sendinblue:
  driver = manualroute
  domains = ! +local_domains
  transport = sendinblue_smtp
  route_list = "* smtp-relay.sendinblue.com::587 byname"
  host_find_failed = defer
  no_more

Add a transport to the TRANSPORTSTART Configuration Box:

sendinblue_smtp:
  driver = smtp
  hosts = smtp-relay.sendinblue.com
  hosts_require_auth = smtp-relay.sendinblue.com

Once you have completed, save all changes. Exim will restart automatically and the changes will take effect.

To confirm that the setup is correct, follow the following steps:

1. Send an email from webmail (remember its subject).
2. Login to Sendinblue and proceed to the Transactional tab.
   
3. Click on the logs tab. You should see the email that you sent in step 1.