I have a workaround for sending email to external users via Nintex Workflow (O365)

As you probably already know, you cannot send emails to addresses that are not valid SharePoint users from the Send an Email action. Nintex says this feature is in development, but I found a workaround that we can use until it is done. I thought I'd share.

1. Created a separate list with the following columns
   1. ToAddress
   2. CCAddress
   3. Subject
   4. Body
2. Create a SharePoint 2010 workflow using SharePoint Designer (non Nintex) that automatically runs when a new item is created on that list, and uses the columns as its data to send an email.
3. In the Nintex workflow you want to send an email, use the “Create List Item” action, and populate the columns (either statically or with variables created earlier in the workflow)

This would have to be setup on each site* you needed to do this on, hence its clunky-ness, but only once for each site, as any workflow on that site could use the same list.

*Workflows can’t create list items cross site, I don’t think. There might be a way, but I haven’t found it yet.