Direct Send is a method used to send emails directly to a Microsoft Exchange Online customer’s hosted mailboxes from on-premises devices, applications, or third-party cloud services using the customer’s own accepted domain. It is an advanced method of sending messages to your users and requires proper configuration of SPF, DKIM, and DMARC to ensure messages are delivered successfully.

Many customers do not need to use Direct Send. Because SPF protections do not always outright block messages and sometimes deliver them to the Junk folder, it is possible for malicious senders mimicking Direct Send to have their messages that spoof your own domains land in the Junk folder and potentially confuse your users. Before this rollout, no feature existed to block Direct Send traffic for customers who have no need to use it. We will roll out the new Reject Direct Send setting for Exchange Online.