# Suppression list: the addresses your email must never reach

A suppression list holds the addresses you must never email: unsubscribes, hard bounces and spam complaints. How it works, and how Mimeo enforces it.

**Definition:** A suppression list is the set of addresses an email platform must never send to: people who unsubscribed, addresses that hard-bounced, and people who marked a message as spam.

## How it works

A suppression list sits in front of every send. Before an email goes, the address is checked against it, and a match stops the email, whatever list, segment or automation picked the person. Addresses get on it in three main ways:

- **Unsubscribes.** The person used the link in the footer or the inbox's own unsubscribe button.
- **Hard bounces.** The address doesn't exist, so mail to it can never arrive.
- **Spam complaints.** The person pressed "Report spam".

Sending providers keep lists of their own too. Postmark deactivates an address after a hard bounce or a spam complaint and skips it on later sends. So one suppression can live in two places: your email platform and your provider.

## Why it matters

- **An unsubscribe has to stick.** US law gives you 10 business days to honor an opt-out, and Yahoo asks bulk senders to do it within 2 days.
- **Dead addresses cost reputation.** Mail sent again and again to addresses that don't exist tells inbox providers you don't look after your list.
- **Moving platforms is the risky moment.** A suppression left behind in the old tool becomes a mailable person in the new one.

## How Mimeo handles it

Mimeo keeps the suppression list in your own database. Unsubscribes, hard bounces, spam complaints and provider-side suppressions all suppress the person, each recorded with its source and reason. Bounces and complaints arrive from Postmark or Resend by [webhook](https://mimeohq.com/docs/api/webhooks).

A [built-in suppression guard](https://mimeohq.com/docs/usage/guards#the-built-in-suppression-guard) checks every send, test sends included, and can't be edited or turned off. Even a badly built segment can't mail a suppressed person. With Postmark, Mimeo also writes its unsubscribes to Postmark's list and reads that list back ([Postmark setup](https://mimeohq.com/docs/providers/postmark#suppression-flows-both-ways)). Resend has a suppressions API too, but Mimeo doesn't read or write it: Resend's bounces and complaints reach your Mimeo by webhook, and Mimeo's own check does the job.

Two habits keep the list whole. When you migrate, [import suppressions first](https://mimeohq.com/docs/usage/migrating#export-everything-including-the-opt-outs), as already unsubscribed. When you erase someone, leave the [suppression stub](https://mimeohq.com/docs/usage/people#erase-person) on, so a later import can't bring them back as a fresh subscriber. See [Mimeo terms](https://mimeohq.com/docs/glossary#sending--deliverability) for suppression in Mimeo's own words.

## Sources

1. [CAN-SPAM Act: a compliance guide for business](https://www.ftc.gov/business-guidance/resources/can-spam-act-compliance-guide-business), checked 2026-11-09
2. [Sender best practices](https://senders.yahooinc.com/best-practices/), checked 2026-11-09
3. [What are bounces and spam complaints?](https://postmarkapp.com/support/article/what-are-bounces-and-spam-complaints), checked 2026-11-09
4. [Guards](https://mimeohq.com/docs/usage/guards), checked 2026-11-04
5. [Webhooks](https://mimeohq.com/docs/api/webhooks), checked 2026-11-09
6. [Postmark setup](https://mimeohq.com/docs/providers/postmark), checked 2026-11-09

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This is the Markdown copy of https://mimeohq.com/glossary/suppression-list.