Email suppression lists

Overview

You send email through an email service provider (ESP), like Mailgun or a custom SMTP server. ESPs keep suppression lists to ensure they don’t send messages from servers that rejected them before or from people who explicitly do not want messages from you.

An ESP suppresses an email in the following situations:

A message experiences a hard bounce. This means that either the recipient email address does not exist or the recipient email server blocked delivery.
A person lodges a spam complaint against a message.

Where you manage email suppressions depends on your email service provider.

If Customer.io is your ESP, you view and manage the list in Customer.io in Workspace settings > Email > Suppression list.
If you send through your own custom SMTP server, your provider keeps the suppression list, and you can only view or change it in your provider’s tools.

Suppressing an email address is different from suppressing a person, which is often done to follow GDPR and other compliance standards.

Email suppression only stops email to one address; the person still exists in your workspace and can receive messages from other channels.

Suppressing people, which is often done for GDPR compliance, deletes the person and blocks all of their identifiers, not just email. You can’t message this person any further.

See Delete or suppress people for a more thorough comparison of deleting, GDPR suppression, and email suppression.

Understand billing

For billing purposes, any message sent to our email service provider (ESP) counts towards billable messages. That includes messages that ultimately bounce, are suppressed, or are marked as spam.

How it works: Customer.io as ESP

When you use Customer.io as your email service provider (ESP), we suppress email addresses that experience a hard bounce, spam complaint or were suppressed through our Email suppression endpoints.

Customer.io checks your email suppression list before sending and records a message to a suppressed address as not sent, with the reason that the recipient is on a suppression list.

If you have multiple sending domains, note that suppressing via API has a wider impact than suppressions that occur due to message delivery:

flowchart LR c{"Is the email address suppressed?"} -- yes, using ESP Suppression API --> f["Person won't receive email from any sending domain in that workspace"] c -- yes, due to hard bounce or spam complaint --> h["Person won't receive email from domain the message came from"] c -- no --> g["Person can receive emails"]

Suppression based on API

When you suppress an address through the Email suppression endpoints, the suppression applies to every sending domain in the workspace. The address can’t receive any email from the workspace.

However, if you unsuppress an email address, you can resume sending emails to the address.

Suppression based on message delivery

A hard bounce or a spam complaint suppresses the address on the sending domain the message came from, in every workspace that uses that domain in your account.

For example, if you send from sketcher.io in both Workspace A and Workspace B, and a hard bounce occurs from a message in Workspace A, we won’t send emails from sketcher.io to that recipient in either workspace. The address can still receive email from your other sending domains, like brandkits.io or mail.sketcher.io. If you send transactional messages from a different domain, this address could still receive order receipts.

Check why you can’t email someone

If you manage email deliveries through Customer.io, you can check if you can’t email someone due to email suppression from Workspace settings > Email > Suppression list.

If you use a Custom SMTP server to send emails, you have to log in there to check your email suppression list; Customer.io does not surface suppression lists managed by custom SMTP servers.

If someone isn’t on your email suppression list and you can’t message them, check whether they were suppressed for GDPR compliance. Go to Workspace settings > Workspace suppressions, and search for them under GDPR suppression list removal.

Remove emails from suppression list

When you use Customer.io as your ESP, you can remove an email address from your email suppression list in workspace settings:

Go to Workspace settings > Email > Suppression list.
Filter for and select the email.
Click Unsuppress.

When you unsuppress an email address, the only change is that the address can receive email again.

To free up the identifiers of a person you deleted and suppressed for GDPR compliance, learn more in Delete or suppress people.

Manage email suppressions via API

If you use Customer.io as your email service provider (ESP), you can manage your suppression list with our Email suppression endpoints.

Look up a single address to see why it was suppressed.
List every address suppressed for a given reason (bounces, blocks, spam_reports, or invalid_emails)
List the bounces and spam complaints for a single sending domain.
Suppress an address. This blocks emails to the address. It doesn’t delete or suppress the person in Customer.io, so they can still receive other kinds of messages, enter automations, etc.
Unsuppress an address from your email suppression list.

When you suppress or unsuppress an address, we apply the change to every sending domain in the workspace. This is different from message-delivery suppression, which only covers the domain that sent the message.

The Email suppression endpoints suppress an email address only. It never deletes a person, unlike the Track API suppress endpoint, which deletes the person and blocks all of their identifiers.

Remember that these endpoints only work when Customer.io is your ESP; you can’t manage email suppressions for your custom SMTP servers with these requests.

Keep suppressions in sync across your sending domains

When you use Customer.io as your email service provider (ESP), a bounce or spam complaint suppresses the address on the domain where it occurred. If you send from more than one domain, your other domains can still email that address.

For example, if you send order receipts from myorders.com and a loyalty program from myordersandrewards.com, an address that hard-bounces on myorders.com could still get sent a message from myordersandrewards.com, which would probably bounce again.

The Email suppression endpoints can help you keep suppressions in sync across your sending domains. When you suppress an address through the API, we apply it to every sending domain in the workspace.

To set a suppression from one domain on others:

Get the suppressed addresses on each of your sending domains, once for bounces and once for spam_reports.
Suppress each of those addresses through the API. Now you can’t send emails to this address across any of your sending domains, not just the one that experienced the complaint.

Updated November 10, 2026