Outbound delivery pools


The improved Microsoft 365 security center is now available in public preview. This new experience brings Defender for Endpoint, Defender for Office 365, Microsoft 365 Defender, and more into the Microsoft 365 security center. Learn what's new. This topic might apply to both Microsoft Defender for Office 365 and Microsoft 365 Defender. Refer to the Applies To section and look for specific call-outs in this article where there might be differences.

Applies to

Email servers in the Microsoft 365 datacenters might be temporarily guilty of sending spam. For example, a malware or malicious spam attack in an on-premises email organization that sends outbound mail through Microsoft 365, or compromised Microsoft 365 accounts. Attackers also try to avoid detection by relaying messages through Microsoft 365 forwarding.

These scenarios can result in the IP address of the affected Microsoft 365 datacenter servers appearing on third-party blocklists. Destination email organizations that use these blocklists will reject email from those messages sources.

High-risk delivery pool

To prevent this, all outbound messages from Microsoft 365 datacenter servers that's determined to be spam or that exceeds the sending limits of the service or outbound spam policies are sent through the high-risk delivery pool.

The high risk delivery pool is a separate IP address pool for outbound email that's only used to send "low quality" messages (for example, spam and backscatter). Using the high risk delivery pool helps prevent the normal IP address pool for outbound email from sending spam. The normal IP address pool for outbound email maintains the reputation sending "high quality" messages, which reduces the likelihood that these IP address will appear on IP blocklists.

The very real possibility that IP addresses in the high-risk delivery pool will be placed on IP blocklists remains, but this is by design. Delivery to the intended recipients isn't guaranteed, because many email organizations won't accept messages from the high risk delivery pool.

For more information, see Control outbound spam.


Messages where the source email domain has no A record and no MX record defined in public DNS are always routed through the high-risk delivery pool, regardless of their spam or sending limit disposition.

Bounce messages

The outbound high-risk delivery pool manages the delivery for all non-delivery reports (also known as NDRs, bounce messages, delivery status notifications, or DSNs).

Possible causes for a surge in NDRs include:

  • A spoofing campaign that affects one of the customers using the service.
  • A directory harvest attack.
  • A spam attack.
  • A rogue email server.

All of these issues can result in a sudden increase in the number of NDRs being processed by the service. Many times, these NDRs appear to be spam to other email servers and services (also known as backscatter).