Out of Office sent via High Risk Delivery Pool
I have been working with 365 Support on this issue, and they instructed I open a Design Change Request as it is currently functioning as designed. Here are the facts regarding Out of Office & High Risk Delivery Pool as they currently function: By design, Exchange Online Protection uses the high risk delivery pool (HRDP) to send OOF replies, because OOF replies are lower-priority messages. From https://learn.microsoft.com/en-us/exchange/troubleshoot/email-delivery/understand-troubleshoot-oof-replies#what-are-out-of-office-replies 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.) 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. 2 & 3 From https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/high-risk-delivery-pool-for-outbound-messages?view=o365-worldwide The ultimate problem / Change Request is: OOF replies SHOULD NOT be placed in the same category as spam / malware / backscatter. Microsoft has already identified a need for a delivery mechanism BETWEEN the normal delivery pool and the high-risk delivery pool; which resulted in the Relay Pool. The auto-response messages, while decidedly lower priority than normal mail messages SHOULD NOT be the same priority as spam / malware messages. These ARE important messages that when undelivered are causing delays / miscommunication among employees and those they converse with outside the company. If Microsoft is going to de-prioritize the OOF replies, they should be put in a separate category and NOT alongside spam / malware / etc.