[Solved] SharePoint : The selected user(s) may not be valid on the site this workflow is published on.

Sending emails to external users:

If an email address is skipped by the SharePoint 2013 workflow then, instead of adding the email address directly into the TO/CC list add it using a workflow variable.

Create a workflow variable of type string. Assign the email address as value to this variable. And use this variable in the workflow Email.

Make this variable to return Login Name instead of string.

Also, to send emails to external users use the 2010 template of workflow instead of the 2013.

2013 workflows skips the external users while sending mailers.