If the email is using an Imported List
This error indicates the CSV file for the recipient list is corrupt. The file type may have originally had a different file type, was saved to CSV, but became corrupt in the process. 
  1. Copy the content from the original CSV File
  2. Open a new CSV file and paste the copied content
  3. Save the file as .CSV
  4. Upload the CSV file to NetCommunity
  5. Create a New Email Template and Message using the new CSV File or use a message with a data source of None
If the email is using a Constituent List
  1. Copy the email message and try to send it again