Here's a breakdown of why an email address might be added to the Suppression list:
Hard Bounces:
These indicate permanent delivery failures, such as an invalid email address or a mailbox that no longer exists.
Spam Complaints:
When a recipient marks your email as spam, the ISP flags it, and it automatically adds the recipient to the suppression list.
Unsubscribe Requests:
Recipients who unsubscribe using the provided link or by using the List-Unsubscribe header are added to the list.
Contact Support to verify and remove emails from this list.