Knowledge base


Suppressions is the list of email addresses that have been rejected during the sending process. 

MailerSend’s suppression list collects bounces, spam complaints, unsubscribes and a custom blocklist. 

Bounces, spam complaints and unsubscribes are populated automatically while sending emails:

  • If the email address doesn’t exist, it’s automatically assigned to Bounces.

  • If a recipient marks your email as spam (for ISPs that provide FBL), the recipient's email address is added to the Spam complaint list.

  • The Unsubscribe list contains all email addresses that manually unsubscribed from your future mailing through the unsubscribe option in your emails. 

Blocklist is a manual list where you can add email addresses manually, or block email sending for recipients using custom patterns.

Please note: The suppression list is separated by domain. Unsubscribed or bounced email addresses on one domain can be active when sending from another domain, until it bounces or the recipient itself unsubscribes.


Bounced emails are addresses from recipients that could not be delivered successfully. For various reasons, the recipient’s server will return the email to the sender—hence the term bounce. 

All bounces can be divided into two main categories: soft bounce and hard bounce. 

The hard bounce list stores email addresses that couldn’t be delivered due to permanent recipient mailbox errors, such as a non-existent mailbox.

Soft bounces, non-permanent errors like when a recipient’s mailbox is full, and other failures (for example, the ISP rejects an email because it sees it as spam) are not added to the list.

Spam complaints

The complaint list stores email addresses of recipients who marked your messages as spam, and applies to ISPs that support feedback loops. A feedback loop (FBL) is a service offered by certain ISPs, where spam complaints are reported back after a recipient marks the email as spam or selects the junk button in their inbox. Below you’ll see which ISPs offer spam complaint information.

Supported by Return Path:

  • BAE Systems (

  • Bluetie/Excite

  • Comcast

  • Cox

  • Fastmail


  • Libero (Italiaonline)

  • Liberty Global (Chello, UPC and UnityMedia) (use the Universal Feedback Loop to apply)

  • Locaweb


  • OpenSRS/Hostedmail (Tucows)

  • Rackspace

  • Seznam

  • Synacor

  • Telenor 

  • Telstra 

  • Terra

  • UOL

  • Virgilio

  • XS4ALL

  • Yandex

Other ISPs that offer FBL:

  • Verizon Media (AOL and Yahoo!)

  • Earthlink

  • (Hotmail)

  • QQ (Tencent)

  • Zoho

Gmail also offers FBL but this service is only available for ESPs, such as MailerSend. Gmail users that mark emails as spam won’t appear in the spam complaint list.


The unsubscribe list contains email addresses of recipients who unsubscribed from your emails by clicking an unsubscribe link in your email (when this option is enabled) or through the List-Unsubscribe header. Click here to read more about unsubscribe tracking. 

When unsubscribe is enabled, you don’t have to worry about whether someone who unsubscribed will receive your email in the future. Unsubscribed recipients are stored in a suppression list.

You can always double-check unsubscribed users. If needed, unsubscribed recipients can be activated manually, but we highly  recommend not using this action.


The blocklist stores email addresses or custom patterns that are added manually. 

For example, you can add specific email addresses to the blocklist that are neither bounced or unsubscribed but that you do not want to send emails to. 

Furthermore, this list can also include patterns. For example, you can block any domain.tld or name. When a pattern is added and a match if found during a sending, the recipient that matches this pattern will be rejected.

How to manually update the suppression list

As mentioned, the suppression list fills-up automatically when sending emails through MailerSend. However, there can be situations where you need to update the list manually, for example when you switch providers.

To update a list, go to Suppressions and choose the list you want to update. Then click on the Add button. Now you’ll see the pop-up window below.

Suppressions page in MailerSend with pop-up window to add emails.

If you have more than one domain in your account, you need to choose the domain first.

To make updates, you can upload a list from a file (we accept .csv or .txt files) or add addresses manually. In the uploaded file, the email addresses should be in the same columns but on different rows. When adding an email list manually, email addresses should be written on separate lines.

If you choose to update the blocklist, there’s an additional option to add a pattern. For example, if you enter pattern "testing", emails like testing@domain.tld or email@testing.tld will be rejected during sending. If you want to block a specific domain, you can add it as @domain.tld.

Need more info?

Please feel free to contact us at and a member of our support team will gladly assist you.