Can you add a custom email header to a Nintex for O365 Workflow Email

I'm and working on a few workflows for O365 that send out email notifications and I'm running into an issue. We currently flag all external emails and place a "This is an external email" heading about them. This allows users to know the email came from an external source so they will verify the email isn't phishing. However, since SharePoint Online is considered an external source, it gets tag as well. We don't want to exclude SharePoint or Nintex entirely because they come from generic "No-reply" emails and you cannot set the "From" field like you can On-Premise. Is there a way to add a custom header to the email (i.e. X-Custom-Header: abc123)? This would allow us to exclude that header from the external email rule. Also, if anyone has other suggestions those are welcomed as well. Thanks in advance!