To configure CPanel to utilize the Sendmetric smarthost you will need to login to WHM and go to the Exim Configuration Manager. Then select the Advanced Editor and make the changes below. 

Before doing this make sure to go the Backup tab and save a backup copy of your Exim configuration in case you have problems.

Section: AUTH

Go to this section in the advanced editor and enter the information below in the text area. Make sure to replace the username and password with the SMTP user that you added. For more information on adding a user see the Getting Started Guide.

#SENDMETRIC_AUTHENTICATORS_START

sendmetric_login:

  driver = plaintext

  public_name = LOGIN

  client_send = : username : password

#SENDMETRIC_AUTHENTICATORS_STOP

Section: POSTMAILCOUNT

In this section add the information below in the text area:

#SENDMETRIC_POSTMAILCOUNTSTART

remoteserver_route:

driver = manualroute

transport = sendmetric_smtp

domains = !+local_domains

route_list = * smtp.sendmetric.com::25 randomize byname

#SENDMETRIC_POSTMAILCOUNTSTOP

Section: TRANSPORTSTART

In this section add the information below in the text area:

#SENDMETRIC_TRANSPORTSTART

sendmetric_smtp:

driver = smtp

hosts_require_auth = *

tls_tempfail_tryclear = true

headers_add = X-AuthUser: ${if match {$authenticated_id}{.*@.*}\

{$authenticated_id} {${if match {$authenticated_id}{.+}\

{$authenticated_id@$primary_hostname}{$authenticated_id}}}}

#SENDMETRIC_TRANSPORTSTOP

Once you make the changes above, scroll to the bottom of the advanced editor and click 'Save'. Verify the Exim service restarts correctly and you are ready to use the Sendmetric service.