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What are email suppression lists & why they’re important

Reading Time: 7 minutes

A well-maintained email suppression list prevents your campaigns from reaching unsubscribed, bounced, and problematic addresses that damage sender reputation. 

For instance, you can stop emails from hitting invalid addresses, respect unsubscribe requests, and avoid costly spam complaints that impact deliverability. 

Email platforms use suppression lists to exclude contacts from email campaigns, keeping your marketing budget focused on reaching engaged subscribers.

Read on to discover how to implement and automate suppression lists to protect your email marketing performance.

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What is an email suppression list?

An email suppression list records addresses that shouldn’t receive communications from your email marketing activities. It helps you maintain clean subscriber data and avoid sending emails to disengaged contacts, invalid addresses, and opt-outs.

Your suppression list will typically include unsubscribed email addresses, hard bounces, and addresses with spam complaints. 

The difference between an email suppression list and your regular one is that suppressed addresses don’t receive future email communications. 

The alternative to a suppression list is segmenting low-quality contacts and unsubscribing them from your list. Omnisend lets you set up rules and triggers to automatically filter low-quality subscribers from high-quality ones to maintain a clean list.

Email suppression list: contacts cleaning

What is an email suppression list used for?

A suppression list in email marketing prevents emails from reaching unsubscribed, bounced, invalid, and potentially harmful addresses. They do this by:

Ensuring compliance

Your suppression list helps you comply with relevant email marketing laws, including GDPR, CAN-SPAM, and CASL.

These regulations require you to honor opt-out requests promptly within specified business timeframes. Not maintaining proper suppression lists can result in substantial fines and penalties under multiple regulations, which vary by region and violation severity.

Protecting sender reputation

Email providers track when subscribers mark your messages as spam, calculating complaint rates that affect your sender score.

High complaint rates make future emails more likely to land in spam folders. A suppression list prevents repeat complaints by automatically stopping sends to these addresses and maintaining your reputation with providers.

Preventing spam traps

ISPs and anti-spam organizations use spam traps via bogus email addresses to identify and block suspicious email senders. 

While it isn’t possible to identify spam trap emails, you can segment your audience and use list tools to remove old and non-engaged contacts to protect against recycled traps (abandoned addresses repurposed by providers).

Enhancing campaign performance

Your email metrics, including open and click rates, improve when you stop sending to uninterested contacts, bounced addresses, and invalid addresses.

Removing chronic non-openers from your active sending list delivers more accurate open and click rates. You can use this data to understand how your audience responds to campaigns and target subscribers who want your content.

Saving on sending resources

Email service providers and marketing platforms typically charge based on the size of the contact database or the number of emails sent.

Maintaining a clean suppression list eliminates costs for emails and contacts that won’t deliver or engage, focusing your resources on reaching active, interested subscribers who are more likely to convert and drive your ROI. 

Types of email suppression lists

There are four main types of email suppression lists:

1. Hard bounces

A hard bounce occurs when your email fails permanently due to invalid addresses, non-existent domains, or server blocks. Your email platform automatically adds these addresses to suppression lists since sending to them damages deliverability and wastes resources.

2. Spam complaints

When subscribers mark your emails as spam, their addresses automatically enter your suppression list. Email providers monitor these complaint rates closely, and continuing to send to these addresses seriously impacts your ability to reach other subscribers’ inboxes.

3. Unsubscribes

Once subscribers opt out of your emails, you must add their addresses to your suppression list and stop sending within specified timeframes. Regulations, such as GDPR, require prompt removal of unsubscribed addresses, with penalties for non-compliance varying by region.

4. Blocklists

A blocklist lets you manually add specific email addresses or domains you want to exclude from campaigns. You might add competitors’ domains, known spam addresses, or contacts who’ve requested removal through channels outside your email platform.

How suppression lists work

Suppression lists work in three ways:

1. Automating suppression list management

Some email marketing platforms automatically add addresses to suppression lists via real-time campaign performance monitoring.

For instance, your list gets updated when subscribers mark messages as spam, click unsubscribe, or the email bounces

Omnisend manages suppressions through contact status segments rather than traditional suppression lists for each channel. It manages subscription statuses separately for email, SMS, and push messages, automatically preventing campaigns from reaching unsubscribed contacts.

It tracks various unsubscribe reasons, from personal preferences and bounces to spam reports, helping you maintain a compliant and up-to-date contact list. 

2. Manual management

Manually managing your contact lists could involve editing them in a spreadsheet, re-uploading them, or editing them within the tool’s dashboard.

During list cleaning, Omnisend reveals categories of “poor-quality contacts,” “disposable, unknown, or unidentified contacts,” and “good-quality contacts,” allowing you to unsubscribe or tag each group strategically.

Some platforms also provide automated segmentation to facilitate manual management. With Omnisend, you can create segments to identify inactive contacts based on engagement timeframes — 30 days for daily sends, 90 days for weekly, or six months for monthly campaigns. 

You can then bulk unsubscribe these addresses rather than deleting them to preserve contact history while cleaning your lists. 

3. Importing suppression lists

You can transfer your suppression history when migrating to a new email platform, although different platforms have different approaches.

Omnisend lets you map opt-out dates during import to maintain unsubscribed statuses. If opt-out dates aren’t available, you can create a separate segment of unsubscribed contacts and update their status after import. 

Omnisend treats bounced addresses and spam reports as unsubscribed contacts. It removes contacts that unsubscribe manually, bounce, and report your emails as spam, helping to keep your list clean with minimal input. 

How to import suppression lists

To import a suppression list to Omnisend:

  • Prepare your suppression list in a CSV file:
    • Use .csv, .xls, .xlsx, or .ods format
    • Ensure email addresses are in a single column
    • Include an opt-out date if possible (in ISO 8601 format)
  • Import process:
    • Go to the Audience tab
    • Click the Add contacts button
    • Select Import contacts from a file
    • Choose your prepared file
How to import suppression lists
Image via Omnisend
  • Map properties:
    • Select email column
    • Map opt-out dates (if available)
    • Choose contact status as non-subscribed
Email suppression list: uploading a contact file
Image via Omnisend
  • Organize contacts:
    • Check relevant subscription status boxes
    • Add relevant tags if needed — when you add a tag, all file contacts are tagged so you can filter and segment them in the future
organizing imported contacts
Image via Omnisend
  • Review and complete:
    • Click Import 
    • Check the Import tab for progress
    • Click View Import Details to see:
      • Number of new contacts added
      • Number of existing contacts
      • Good and bad email/phone number counts
Email suppression list: imported audiences
Image via Omnisend

That’s it! Your list is now imported into Omnisend. 

Pro tip: Omnisend automatically manages unsubscribed contacts, including those who bounce or mark emails as spam.

Use Omnisend’s powerful automation tools to maintain your email lists

How to manage suppression lists effectively

Follow these five steps for effective email suppression list management:

Regular monitoring

Track your campaign metrics to identify patterns in unsubscribe reasons, bounce rates, and spam complaints. Your monthly reviews will catch potential issues before they grow.

For instance, segments showing high bounce rates point to data quality problems. Spam complaint clusters usually mean your content or targeting needs adjustment. 

Timely monitoring lets you maintain a healthy suppression list, improving deliverability and engagement across your campaigns.

Using email verification tools

Email verification tools, like Bouncer, ZeroBounce, Emailable, etc., can catch invalid addresses before sending campaigns. Scanning for syntax errors, defunct domains, and spam traps protects your reputation. Running verification checks before email campaigns saves resources and improves delivery rates.

A clean list means better engagement and fewer bounces. Good verification practices help you maintain professional sender status with email providers.

Implementing double opt-in

Start your relationship with subscribers through double opt-in confirmation links. You’ll capture genuinely interested contacts while filtering out typos and fake addresses.

Making subscribers verify their interest leads to higher engagement rates. Double opt-in might slow initial list growth but ensures quality contacts. Your long-term metrics will benefit from having an engaged, verified subscriber base.

Using rules to add specific types of emails automatically

Set automation rules to filter risky addresses before they enter your lists. Your filters could flag free email domains in B2B campaigns, catch disposable services, and spot major domain typos.

You could also create segments to unsubscribe contacts automatically. Omnisend has pre-built segments that identify disengaged subscribers, helping you save time and stay organized. These include segments for tracking inactive subscribers and lapsed email subscribers. 

Sync databases with other apps

Email marketing often requires multiple tools, including customer relationship management (CRM) and other email marketing platforms. Syncing databases across your apps prevents sends to suppressed addresses and unifies your data. 

For example, Retention helps recover abandoned website visitors by identifying and contacting them through email. It checks your Omnisend database — including active subscribers, unsubscribes, and bounced addresses — to suppress contacts in your Omnisend account.

Check out Omnisend’s other integrations to find your platform.

Conclusion

Adding unsubscribed, bounced, and low-quality addresses to an email suppression list can improve deliverability and engagement rates, helping you maintain your sender reputation and achieve a bigger return on investment from email marketing

Another approach to cleaning your email list is setting rules in Omnisend to automatically unsubscribe low-quality contacts (for instance, those that aren’t valid or haven’t engaged with your brand for 90 days) to create contact status segments.

Whichever route you choose, remember to actively manage your contact list to ensure you’re communicating with your most engaged and valuable subscribers. 

Monitor your deliverability metrics and open rates regularly, using this data to refine your segmentation strategy and optimize campaign performance. 

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Aistė Jočytė
Article by

Aiste is a Jr. Content Marketing Manager at Omnisend. When she's not searching for the perfect synonym or refining her latest copy, you can find her curled up with her cat, binge-watching yet another TV series.