High-risk delivery pool for outbound messages
When a customer's email system has been compromised by malware or a malicious spam attack, and it's sending outbound spam through the hosted filtering service, this can result in the IP addresses of the Office 365 data center servers being listed on third-party block lists. Destination servers that do not use the hosted filtering service, but do use these block lists, reject all email sent from any of the hosted filtering IP addresses that have been added to those lists. To prevent this, all outbound messages that exceed the spam threshold are sent through a high-risk delivery pool. This secondary outbound email pool is only used to send messages that may be of low quality. This helps to protect the rest of the network from sending messages that are more likely to result in the sending IP address being blocked.
The use of a dedicated high-risk delivery pool helps ensure that the normal outbound pool is only sending messages that are known to be of a high-quality. Using this secondary IP pool helps to reduce the probability of the normal outbound-IP pool being added to a blocked list. The possibility of the high-risk delivery pool being placed on a blocked list remains a risk. This is by design.
Messages where the sending domain has no address record (A record), which gives you the IP address of the domain, and no MX record, which helps direct mail to the servers that should receive the mail for a particular domain in the DNS, are always routed through the high-risk delivery pool regardless of their spam disposition.
Understanding Delivery Status Notification (DSN) messages
The outbound high-risk delivery pool manages the delivery for all "bounced" or "failed" (DSN) messages.
Possible causes for a surge in DSN messages include the following:
A spoofing campaign affecting one of the customers using the service
A directory harvest attack
A spam attack
A rogue SMTP server
All of these issues can result in a sudden increase in the number of DSN messages being processed by the service. Many times, these DSN messages appear to be spam to other email servers and services.