Suppression list: the addresses your email must never reach

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.

A 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). 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, as already unsubscribed. When you erase someone, leave the suppression stub on, so a later import can't bring them back as a fresh subscriber. See Mimeo terms for suppression in Mimeo's own words.