This is doable in CRM from Administration>Email Services>Blacklisted Email Addresses tab. 

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.