Issue Error Messages: 

Task ‘Mailer – Sending’ reported error (0x800CCC0F) : ‘The connection to the server was interrupted. If this problem continues, contact your server administrator or ISP. The server responded: 501 Too many syntax or protocol errors’

 Common Causes for the error include:

• A misspelled email address in the To: field (most common)
• Improper Characters in the To: field Eg: < > [ ] etc.
• Leaving a space before or after the email address
• Manually typing the wrong SMTP commands into a terminal window
• A mail filtering or mail scanning plugin which is integrated into your mail client Eg: An Anti-Virus or Firewall
• An old / incorrect message lying in your Outlook Outbox folder.

Solution:
When sending emails using outlook you might receive the error above. It seems that when you copy & paste the email addresses into the To field, they include quotations around them that the server did not know how to handle (i.e. ‘name@website.com’ instead of just name@website.com).

Remove the quotations & then try sending emails.