When a mailbox provider hard-bounces an address, or a recipient marks your mail as spam, sending to that address again doesn’t just fail—it actively damages your domain reputation. A suppression list is the system that remembers so your send logic doesn’t have to.
How it works
Addresses land on the list from three origins: bounce (the receiving server permanently rejected the mail), complaint (the recipient hit the spam button), and manual (you added it yourself—an angry customer, a legal request, a known trap). From then on, sends to that address are skipped automatically. On Resend the list is native, team-wide across all your domains and subdomains, visible in the dashboard, and manageable via API—including bulk import if you’re migrating from another provider.
The rules of engagement
- Never mail around it. Exporting your list to another tool to reach suppressed addresses is how senders convert one bounce into a reputation incident. The suppression is protecting you, not inconveniencing you.
- Be very slow to un-suppress. Removing an address without fixing the reason it was suppressed just replays the damage—it will bounce or complain again, get re-suppressed, and cost you twice.
- Mine it for signal. A bounce spike traces to a stale segment or a broken signup form; a complaint cluster traces to a campaign that outran consent. Wire up
email.bouncedandemail.complainedwebhooks and the list becomes telemetry, not just a blocklist. - Mind the Gmail gap. Gmail doesn’t report individual complaints back to senders, so complaint-origin suppressions undercount your Gmail audience—Postmaster Tools covers that blind spot.
The verdict logic
If your ESP is any good, you need a suppression list in the sense that you need brakes: it’s already installed, and the failure mode is overriding it. If you’re building sending infrastructure yourself, this is one of the systems you now owe the world—which is its own answer.