This error can occur due to invalid formatting of email addresses in an imported list. Additionally, special characters can contribute to this as special characters are not supported in an imported list. If this error occurs perform the following steps to resolve: 
  1. Ensure that the email address formatting does not contain any www. or special characters
    • Example of proper formatting: test@test.com
    • Example of incorrect formatting: www.test@test.com
  2. Once all email address formatting is verified, make sure that any special characters are removed from the CSV file
    • Special characters can be any of the following: !, #, $, %, ^, &. *, (, ), +, [, {, }, ], <, >, ?, '
If the error is still occurring after removing all special characters and verifying the email formatting, review our formatting guide.