Typically, this error will prompt when an invalid URL is added to the email message content. To resolve, you'll need to view the HTML and remove any invalid links. The HTML can be viewed via:
  1. While editing the email message, navigate to the View tab
  2. Click the HTML code button
  3. Review the content for any invalid URLs and remove them
User-added image
  1. When finished, click Update
  2. Click Save