Looks simple enough but it took a few tries to get the SMTP servings right for a “Google for Domains” account:
Remember that Google limits each mail account to sending 500 messages a day to foil spammers. If you have a higher volume than that, then you will have to look elsewhere for an SMTP server.
As a bonus, when you use GMail for your SMTP server you can see the history of emails sent from DotNetNuke in your GMail Sent folder. This really helps with debugging email notification issues.
For example, I could never figure out where the Test messages were going until I saw them in my Sent Folder. By looking in the Sent folder, it was very obvious that they are sent to the Host’s Email address which I still had set to the default value of support@portal_domain.com.
Let me know if this helps you out.