Suppression List: A suppression list contains the email addresses of people who have either registered to avoid or opt-out of receiving emails after signing up for them. This is also sometimes referred to as an email suppression list.

In order to comply with CAN-SPAM, which regulates how businesses can send promotional emails, companies must have a visible and functional "Unsubscribe" link in all emails. This gives users the chance to leave a mailing list at any time. Businesses have a certain amount of time (typically ten days) to remove subscribers from that mailing list after their request. These users are then added to that suppression list.

Rather than removing the addresses from the list completely, though, a suppression list is simply another list that blocks future messages from being sent. In some cases, the suppression list can be taken over by a third party, who will use the list to send emails to those addresses. This is referred to as suppression list abuse.

However, there are ways to protect and encrypt suppression lists to prevent abuse. Having a neutral third party "scrub" the lists is one method. Another is to use a form of encryption known as a cryptographic hash function, which disguises the addresses in code. This type of code only goes one way -- so even if another party has the code, it can't be "translated" back to the original email address.