Email Suppression Lists

On April 15, 2022
6min read
Grigori Monaselidze Content Manager @ Mailtrap

What is an Email Suppression List?

Suppression Lists have been around for nearly 20 years. This tool allows businesses to greatly improve their sending in terms of delivery and rapport. The implementation of Suppression Lists was partially enforced by the CAN-SPAM Act of 2003 which requires senders of commercial or promotional emails to provide a functional unsubscription option. 

To simplify, a Suppression List represents a list of addresses that should not receive emails from a sender. Otherwise, the domain is at risk of being blacklisted by powerful Internet Service Providers (ISP), such as Google.

Suppression Lists can be one of the most effective tools to improve your senders’ reputation, which directly benefits your business.

How to work with Suppression Lists?

Suppression Lists’ functionality is fairly simple. Most emails reach your businesses’ Suppression List automatically when:

  • A client unsubscribes from your emailing services;
  • A client is unable to receive your emails;
  • A client reports your emails as spam.

We’ll elaborate on these further down the article. For now, it is worth noting that most Suppression Lists get populated automatically, and there are manual management options as well. Here’s how it goes.  

Once an email campaign begins, the system compares the recipient addresses to those present in your Suppression List. The addresses in the list are suppressed during the campaign, but you can manually type an address to add to your Suppression List.

Email Types in Suppression Lists

These are the emails that would reach your Suppression List either automatically or manually. Check the specifics of each type below.


Unsubscribed emails are common in Suppression Lists. These are the recipients who choose not to receive your emails.

They are typically added automatically if the unsubscribe function performs properly. Sometimes the clients will request the company to unsubscribe them from the mailing list. They can do it directly or using an unsubscribe app, after which their addresses must instantaneously and manually be added to the Suppression List. 


Unwanted emails could be spam complaints or reports toward the content that you have sent to the recipients. This could happen automatically when a system detects your email as spam due to its erroneous wording or contents. Or, it could be manual – i.e., reported by a recipient. 

With an automatic spam report, the email could be changed and adapted to the standards of the ISP. But if your email was manually reported as spam, you must not email those recipients. Sending out emails to them puts your domain or IP address at a risk of being deny-listed, which is detrimental to your sending reputation.


These are the email addresses that have a Hard Bounce status on them, which happens for various reasons. The email address simply doesn’t exist, the recipient’s inbox is full, or the domain is no longer active. Or, there is some sort of trouble on the recipient’s server 


In other words – bounced. These emails can either be identified as soft or hard bounces. An email bounces when the address returns the email back to the sender. This could either mean that the address is incorrect or the server is inactive. 

A soft bounce is a temporary failure of email delivery, while a hard bounce is permanent. Ideally, bounced emails are automatically added to the Suppression List to avoid further sending. This is healthy for your deliverability statistics, and it convinces your ISPs that you take care of your contact list and are a trustworthy sender.

Why are Suppression Lists important?

Suppression Lists are mainly used so businesses comply with the CAN-SPAM Act and ensure the recipients have an option to unsubscribe. In doing so, the senders warrant a more satisfactory experience for the recipients and a higher deliverability rate for themselves. 

With that in mind, utilizing Suppression Lists could guarantee a greatly improved deliverability. They’re also useful to control the sending frequency and content. All that allows you to have greater control over your sending and helps keep the clients engaged.

Suppression List & Email Deliverability

What is Email Deliverability?

Email Deliverability is used by many companies to measure the ability of their emails to reach subscribers’ inboxes with email campaign Suppression Lists. This measurement allows email marketers to determine the success of their email campaigns. 

Email Deliverability is measured by specific metrics in the email sending software. This allows the senders to accurately measure the delivery rates and detect issues preventing emails from reaching subscribers. Even though the tool does not allow a specific estimation of how many emails reach the general inbox and how many the spam folder, senders are able to make informed references by analyzing the available metrics in the email sending tools.

Various issues could damage the email deliverability. Sending emails from unverified or authenticated domains, with a difficult unsubscription option, shortened URL or single opt-in forms, are among the most common. Aside from these, there are issues outside of businesses’ control, which can be monitored to improve the sending.

Email Service Providers (ESP) are able to show why these deliverability issues occur. This allows the sender to maintain email list hygiene. Bounce rates, open rates, and click rates are all tracked by the ESP, along with unsubscribes and reports. This directly allows the sender to gain insight and handle their content for improved email engagement.

Connection between Email Deliverability & Suppression Lists

As more emails get delivered, more purchases are made, fewer processes are broken and users remain informed.

That’s where the positive impact on your deliverability rate occurs since you will not be sending out any emails to the addresses that do not accept them.

How to manage a Suppression List?

Most email Suppression List management is done automatically. Addresses that are harmful to your email deliverability are added automatically, and therefore, automatically blocked for any future email campaigns.

However, at times, there might come a need to clean and remove an email from a Suppression List that no longer belongs there. For example, a user could have accidentally unsubscribed from your emailing campaigns or simply have changed their mind. It is important to mention, that emails in the Suppression List do not receive transactional emails as well. A user might require such an email and contact your Support Team, asking to be returned to your emailing services.

You can also move users to the Suppression List manually. This is particularly helpful when migrating from one sending solution to another, simply importing CSV to the current sending solution’s Suppression List will automatically transfer the addresses and keep your deliverability rate in a healthy state.

How to minimalize the number of emails in your Suppression List?

You can simply do this by observing the statistics and making appropriate decisions. For example, if emails are mainly reported as spam – they do not comply with the interests of the audience, therefore the content in the emails can be adjusted accordingly. Similar to unsubscriptions, improving your customer relations and retention is key to avoiding a significant number of unsubscribers.

It is highly recommended to focus your email content on existing customers, encouraging their loyalty, and overall improving customer experience. Research shows that loyal customers are 14 times more likely to invest in your services than a new one. However, it is important to note that an audience is not one homogenous group, they all have different interests. Yet with a Suppression List, you will be able to target the most qualified group with personalized content and improve your engagement with them.

Email Validation is also an essential part of maintaining your email list hygiene. There are various alternative solutions to validate an email list, including paid services, however, most modern browsers are able to check the address automatically upon sending.

Simply observe your Suppression List, which addresses land for what reason. Making appropriate decisions depending on those reasons could drastically reduce the number of emails in the Suppression List, and improve customer engagement.

What is CAN-SPAM?

Controlling the Assault of Non-Solicited Pornography And Marketing (CAN-SPAM) is a law that was passed in the United States in 2003, and is enforced by the Federal Trade Commission (FTC). This was the national standard for commercial emailing. This standard enforces the regulation on commercial email marketers to not send misleading, unsolicited, or deceptive content to recipients, while giving them the right to decline such emails. 

CAN-SPAM act is defined by three types of compliances — unsubscribe, content, and sending behavior. All commercial email marketers are legally enforced to adhere to these compliances.

Unsubscribe compliance

  • A visible and operable unsubscribe mechanism is present in all emails.
  • Consumer opt-out requests are honored within 10 business days.
  • Opt-out lists, also known as suppression lists, are used only for compliance purposes.

Content compliance

  • Accurate “From” lines.
  • Relevant subject lines (relative to offer in body content and not deceptive).
  • A legitimate physical address of the publisher or advertiser is present. 
  • A label is present if the content is adult.

Sending behavior compliance

  • A message cannot be sent without an unsubscribe option.
  • A message cannot contain a false header.
  • A message should contain at least one sentence.
  • A message cannot be null.
  • An unsubscribe option should be below the message.

This act legally encourages marketers to be conscientious with their emails and ensure strict compliance with the above-mentioned aspects. However, there can also be fraudulent practices due to misinterpretations that are viewed as criminal offences.

Aside from legal requirements, most companies now comply with the CAN-SPAM Act to have better engagement with their clients, and to also improve their email deliverability.


With Suppression Lists, any businesses’ email campaign performance can be significantly improved. It is specifically useful to target the correct customers to improve retention and communication. This will warrant a successful campaign where your business’s recourses and efforts shall yield beneficial achievements, instead of leaving it to a chance.

Control the satisfaction of your customers by allowing them to adjust the frequency of content they receive from you. This enables your business to solidify a strong sending reputation on the market.

Article by Grigori Monaselidze Content Manager @ Mailtrap