This error can be related to a number of causes. The main cause is that the template you are trying to use to send your email has an issue with its setup. 

To resolve this:

1. Launch Document Template Manager
2. Locate your email template
3. Double click to open
    
     -Check for leading or trailing spaces
    
      - Does either the Name or the email address have any extra spaces before or after it? Remove those, if so:

     -Check your merge fields
       
        - Do any of your merge fields appear to display as blank? If so, remove them