Suppression List

A suppression list is a set of email addresses a sender must never mail again, typically people who unsubscribed, complained, or hard-bounced. Sending tools check it before every send, so suppressed contacts stay excluded even if they reappear in a new import.

Deliverability & Email InfrastructureUpdated September 30, 2026

In short

A suppression list blocks sends to people who opted out, complained or bounced.

Key points

  1. Typical entries are unsubscribes, spam complaints, Hard Bounce addresses and invalid addresses [1].
  2. Under the CAN-SPAM Act, opt-out requests must be honored within 10 business days, and the address may not be sold or transferred [2].
  3. Gmail recommends fulfilling unsubscribe requests within 48 hours, and Yahoo asks senders to honor them within 2 days [3][5].
  4. RFC 8058 One-Click Unsubscribe requests arrive as HTTPS POSTs and should write straight to the suppression list [4].
  5. A suppression list should be global across campaigns, sending domains and mailboxes, so an Opt-Out in one place applies everywhere [1].

What goes on a suppression list

A suppression list records addresses that should be excluded from all future sends, independent of which list or campaign they came from. Email platforms usually keep several categories. SendGrid, for example, separates global unsubscribes, group unsubscribes, bounces, spam reports, blocks and invalid emails [1]. The two most important categories are legal and reputational. Opt-outs are a legal matter: the FTC's CAN-SPAM guidance requires honoring them within 10 business days [2], and privacy laws such as GDPR and CASL add their own obligations. Bounces and complaints are reputational: repeatedly mailing a Hard Bounce address or someone who reported you as spam signals poor list hygiene to providers and raises your Bounce Rate and Spam Complaint Rate.

Why it must be checked on every send

Contacts come back. A person who unsubscribed last quarter may show up again in a new Lead List, a CRM sync or an enrichment export. If suppression depends on the list rather than on a central check, that person gets mailed again, which is both a compliance failure and a likely complaint. The safe design is a single suppression store checked immediately before each message is queued, keyed on normalized email address and, for opt-outs, often on the whole domain when a company asks not to be contacted. Records should be kept indefinitely; removing an unsubscribe after a period defeats its purpose, and CAN-SPAM prohibits using the address for anything other than compliance once someone opts out [2].

Feeding the list automatically

Manual suppression breaks at scale, so most inputs should be automatic. Unsubscribe links and the List-Unsubscribe Header should add the address as soon as the request arrives; Gmail recommends completing removals within 48 hours and Yahoo within 2 days [3][5]. The One-Click Unsubscribe mechanism in RFC 8058 sends a POST request that needs no further confirmation, so it maps cleanly to an automatic write [4]. Bounce processing should add permanent failures, and Feedback Loop (FBL) reports should add complainers. Replies that ask to stop, detected through Reply Detection or read by a person, belong on the list too, even when the recipient never clicked a link.

Sources
  1. Suppressions — Twilio SendGrid Docs
  2. CAN-SPAM Act: A Compliance Guide for Business — Federal Trade Commission
  3. Email sender guidelines FAQ — Gmail Help
  4. RFC 8058: Signaling One-Click Functionality for List Email Headers — RFC Editor
  5. Sender Best Practices — Yahoo Sender Hub
External sources open in a new tab.

Related terms

Opt-Out

Every commercial email needs an easy way to say stop, and once someone does, you must stop emailing them within the legal deadline and keep them suppressed.

Hard Bounce

A hard bounce is a permanent rejection, and the address should never be mailed again.

One-Click Unsubscribe

One-click unsubscribe lets people opt out instantly from the inbox, with no web page or login.

CAN-SPAM Act

CAN-SPAM lets you send commercial email in the US without prior consent, as long as you are honest about who you are, include an address, and honor opt-outs quickly.

Feedback Loop (FBL)

A feedback loop tells senders who marked their mail as spam so those people can be removed.

List-Unsubscribe Header

List-Unsubscribe is a header that lets the inbox itself offer an unsubscribe button.

Mentioned in
AI Sales AgentBounce RateBreakup EmailCASLCatch-All Email AddressCRM (Customer Relationship Management)Data DecayData MinimizationEmail SequenceEmail Service Provider (ESP)Email VerificationGDPRLead ListLegitimate InterestMultichannel OutreachObjection HandlingOutreach CadencePECRPositive Reply RateReply DetectionReturn-PathRight to ErasureSales Engagement PlatformSender ReputationSentiment AnalysisSoft BounceSpam TrapWaterfall EnrichmentWebhookWorkflow Automation

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