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Hi Kevin,
When you add a step “Email these users” in workflow and select type an external email address, you will receive a pop-up stating “The selected user(s) may not be valid on the site this workflow is published on. If a recipient is not a valid SharePoint user, he or she will not receive workflow emails.”
This is because by default SharePoint Designer workflows creating using SharePoint 2013 Workflows will not sent emails to the external users (email addresses).
For detail information, please see SharePoint 2013 – Unable to send emails to external email addresses (SharePoint 2013
Thanks,
Neha