What is a suppression list?

A suppression list is a database of email addresses that must be excluded from all marketing sends, regardless of how or when they might otherwise appear in your mailing lists. When someone unsubscribes, complains, or requests removal from your communications, their address goes on your suppression list to ensure they never receive another marketing message from you. The suppression list acts as a master override-even if that address gets imported into a new list or appears in a purchased data set, the suppression check prevents them from being contacted.

Suppression lists typically include several categories of addresses: unsubscribes (people who opted out through your unsubscribe mechanism), hard bounces (addresses that are permanently undeliverable), spam complaints (addresses that marked your emails as spam), and manual removals (addresses you've added due to legal requests, customer service issues, or other reasons). Some organizations maintain separate suppression lists for different purposes, while others consolidate into a single master suppression. Either way, the principle is the same: these addresses are off-limits for marketing outreach.

Proper suppression list management is both a legal requirement and a deliverability best practice. CAN-SPAM, GDPR, CASL, and other regulations require honoring opt-outs, which practically means maintaining accurate suppression data. From a deliverability standpoint, continuing to send to people who've unsubscribed or complained damages your sender reputation and can lead to blocklisting. Your ESP likely maintains its own suppression list that syncs with your account, but you should also maintain your own master suppression data for cross-platform consistency and data portability. The suppression list isn't where addresses go to be forgotten-it's where they go to be remembered and protected from further contact.