Please separate the recipients on your list by using commas

The error please separate the recipients on your list using commas can occur when multiple email addresses in the To field are not separate with comma (,)

For example: 'instantanswers@netzero.net, macdmand@juno.com, test@mybluelight.com'

Also this error could happen for the reasons listed below:

• The email addresses entered in the To: field could be incorrect. Please check if all the email addresses have the proper format, for ex: yourname@mailserver.com
  
  
• Please review all the Address Book entries that you have created. Please send a mail to each of the addresses individually to see if you receive an error message for any one of the addresses. The address for which you receive an error message is the one in incorrect format.
  
  
• If the option to Automatically Add all Incoming Addresses to the Address Book, is enabled, then the Address Book does not verify the format of these addresses and if any of the addresses are in an incorrect format, they get added to the Address Book, and may cause this error.