How Dynamics 365 Marketing uses suppression lists

A suppression list is a powerful backend tool that ensures your sending reputation stays healthy. Suppression lists prevent email messages from being sent to harmful recipient addresses.

This article discusses types of harmful address, then details how suppression lists prevent your emails from reaching these addresses.

Recipient addresses that affect your sending reputation

Hard-bounced addresses

Sending frequent emails to hard-bounced, invalid email addresses alerts remote spam filters to potential “spammy” behavior. These alerts affect your inbox placement and deliverability results. Sending multiple hard bounces to the same email provider in the same batch may even lead to the email provider completely blocking a sender, which can affect valid recipients.


A spamtrap is a special email address that is used by email providers and email protection systems to identify spammers. A spamtrap address can be specially created or an email address that was disabled because of the owner’s inactivity. Spamtrap addresses are never announced and can't be separated from regular, valid email addresses, making it especially important to avoid sending to them.

Addresses that mark your mail as spam

Another type of address that is important to avoid is an email address that has marked your emails as spam, or, in other words, an email address that has generated complaints. Continuing to send emails to a person who has marked your email campaigns as spam negatively affects your sending reputation and labels you as a spam source.

How the suppression list works

A suppression list is an automated backend tool that protects your sending reputation. The list is divided into three scopes of operation:

Hard bounce suppression

Hard bounced email addresses are collected and stored in the suppression list. The hard bounce portion of the suppression list is a per-organization list. The list stores cumulative information from organization sending statistics, making the list broad and reliable.

When a customer sends an email campaign, the system automatically checks the segment used for the campaign against the list of known hard bounces inside the suppression list and prevents such addresses from going out. Soft bounced email addresses may also be listed in this suppression list. After five sequential failed attempts to deliver to the same email address, soft bounce addresses are converted to hard bounce addresses. More details on bounce reasons and categories can be found here: Email bounce categories.

To keep the list up-to-date, the backend tool stores information about hard bounced addresses for 180 days (about six months). In rare cases, a hard bounced address may become valid again.

Spam complaint suppression

The spam complaint suppression list stores information about spam complaints (feedback loop reports) that were received regarding a specific organization and prevents such addresses from being sent to. Spam complaints are stored only for the specific organization that received a complaint. Spam complaint listings in the suppression list don't expire automatically.

Pattern suppression

The pattern suppression is a manual list maintained by the deliverability engineers. It contains domains or email addresses that may not be a good idea to send to. For example, test mailboxes that will never read emails or domains that host temporary or disposable email addresses. Such mailboxes and domains are accessible to anyone and sending to them may harm your sending reputation.

How to delist email addresses

If you find that some contacts were blocked with a “suppression list” reason and you're sure that they're 100% valid and the listing was caused by false-positive, you can submit a support request, providing the following information:

  • A list of the email addresses in question.
  • Justification as to why you believe each email address was listed by mistake. You should also provide proof that each email address is valid and can be reached by any other email provider.

After that, our deliverability team will review the request and determine if the addresses should be removed.


If a contact is removed from a suppression list that is used in a live journey, the changes will take effect in the next iteration of the journey (if the journey is recurring), not in the current one.

How to prevent email addresses from being listed

To prevent your email addresses from being listed on suppression lists, it's crucial to pay close attention to your email bounce rates. Bounce rates are the number of emails that couldn't be delivered to the recipient's inbox. A high bounce rate can indicate several issues including invalid email addresses, incorrect email formatting, or email content that triggers spam filters. Addressing these issues can help improve your email deliverability rates and prevent your email addresses from being listed on suppression lists.

Here are some tips to avoid email suppression lists:

  1. Maintain a clean email list: Regularly clean your email list by removing invalid or inactive email addresses. It's also essential to validate email addresses before adding them to your list to avoid sending emails to fake or spam email addresses.
  2. Send relevant and valuable content: Deliver emails with relevant and valuable content to your subscribers. This reduces the likelihood of recipients marking your emails as spam.
  3. Test your emails: Test your emails before sending them to ensure that they're well-formatted and don't trigger spam filters.
  4. Monitor your email deliverability rates: Keep an eye on your email deliverability rates and investigate any sudden changes. A sudden drop in deliverability rates could indicate that your emails might have some potential issues.
  5. Follow email marketing best practices: Adhere to email marketing best practices such as including an unsubscribe link in your emails and ensure that your email content complies with anti-spam laws.

It's the sender's responsibility to maintain a clean email list and ensure that their email campaigns comply with email marketing best practices. By following these tips, you can avoid having your email addresses bounced and listed on suppression lists and maintain a healthy email reputation.

See also:

Fix a high email bounce rate (Dynamics 365 Marketing) | Microsoft Learn Email bounce categories (Dynamics 365 Marketing) | Microsoft Learn