A suppression list is a list of email addresses you want to exclude from receiving email. Subscriber email addresses are typically added to a suppression list when they unsubscribe or complain through a complaint feedback loop. Other reasons an email address may be added to a suppression list is when it is an unknown user, the mailbox is always full (indicating the account was abandoned), or if the subscriber is inactive (indicating a loss of interest in receiving your email).
Email Glossary
