Outbound delivery pools

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 block lists. Destination email organizations that use these block lists 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 block lists.

The very real possibility that IP addresses in the high-risk delivery pool will be placed on IP block lists 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).

Relay pool

Messages that are forwarded or relayed out of Microsoft 365 are sent using a special relay pool, since the final destination should not consider Microsoft 365 as the actual sender. It's also important for us to isolate this traffic, because there are legitimate and invalid scenarios for autoforwarding or relaying email out of Microsoft 365. Similar to the high-risk delivery pool, a separate IP address pool is used for relayed mail. This address pool is not published since it can change often.

Microsoft 365 needs to verify that the original sender is legitimate so we can confidently deliver the forwarded message. In order to do that, email authentication (SPF, DKIM, and DMARC) needs to pass when the message comes to us. In cases where we can authenticate the sender, we use Sender Rewriting to help the receiver know that the forwarded message is from a trusted source. You can read more about how that works and what you can do to help make sure the sending domain passes authentication in Sender Rewriting Scheme (SRS).