If your email template contains bracket characters (< or >), this will cause the Unexpected Error message on the last step of the mass email. These characters will need to be removed before you can send the email:
  1. Click Communications in the menu
  2. Click Manage Communications
  3. Click on the category that your template is stored under
  4. Click on the name of the template to open it
  5. Within your template, remove any bracket characters:
Brackets
  1. Click Save And

Once the brackets have been removed, attempt to send the mass email again. If you still receive an error, contact support and reference this article.