SMTP ERROR: Password command failed: 535 Incorrect authentication data - WHMCS - Google Apps Email

A tricky little problem is when installing WHM on a new server, the option “Restrict outgoing SMTP to root, exim, and mailman (FKA SMTP Tweak)” is on by default. To allow your server to send emails using Gmail / Google Apps Email / or another external email provider you will need to turn this option off. Leaving this enabled causes the following error in WHMCS: SMTP Error: Could not authenticate.

Go to "Tweak Settings" in WHM

And search for “Restrict outgoing SMTP to root, exim, and mailman (FKA SMTP Tweak)”

Select "Off"

See the screenshot below:

email settings