After sending an email, email addresses are being added to the Blackbaud blacklist.
Blackbaud takes the proactive anti-spam measure of collecting email addresses on a blacklist. This list is an effort to protect the organization's as well as Blackbaud's reputation as an email sender. It ensures no further email can be sent to the email address that was used.
An email address can be added to the Blackbaud blacklist due to a Spam Complaint: If a recipient marks an email as Spam, their email address will be added to the blacklist on the Blackbaud Services servers.