To configure cPanel emails to relay via Brevo / 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.brevo.com::587 byname"
  host_find_failed = defer
  no_more

Add a transport to the TRANSPORTSTART Configuration Box:

sendinblue_smtp:
  driver = smtp
  hosts = smtp-relay.brevo.com
  hosts_require_auth = smtp-relay.brevo.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 Brevo / Sendinblue and proceed to the Transactional tab.
   
3. Click on the logs tab. You should see the email that you sent in step 1.