To toggle the Blacklisted status of a particular email address:
  1. In CRM go to Administration>Email Services
  2. Select the Blacklisted Email Addresses tab
  3. Find the email address you would like to change 
  4. Select Toggle blacklisted Status 
  5. At the prompt Are you sure you want to change the blacklisted status of this email address?​ select Yes

Note: You cannot remove blacklisted email addresses from the Blacklisted Email Addresses tab. To resume sending email to a blacklisted email address, you can toggle its status. For example, if the Internet Service Provider incorrectly reports an email address as invalid and you want to change its status to "Not blacklisted," select the address under Blacklisted email addresses and click Toggle blacklisted status. On the confirmation message, click Yes. You return to the Blacklisted Email Addresses tab.