If you are setting up external SMTP like Google Workspace and are getting following error:
|SMTP Debug: SMTP Error: Could not authenticate.|
Then you need to do some tweaks inside WHM.
- WHM > Tweak Settings > “Restrict outgoing SMTP to root, exim, and mailman (FKA SMTP Tweak)” (or through WHM > SMTP Restrictions)
- If server uses CSF then ensure that SMTP_BLOCK is disabled in the CSF configuration