Mail flow in EOP
As an Exchange Online Protection (EOP) customer, all messages sent to your organization pass through EOP before your workers see them. Whether you host all of your mailboxes in the cloud with Exchange Online, or you host your mailboxes on premises (called a standalone scenario), perhaps to continue taking advantage of your existing infrastructure, you have options about how to route messages that will pass through EOP for processing before they are routed to your worker inboxes.
You may want to configure custom mail routing to conform your messaging to a business requirement. For instance, you can pass all of your outbound mail through a policy-filtering appliance.
Working with messages and message access options
EOP offers a lot of flexibility in how your messages are routed. The following topics explain steps in the mail flow process.
Use Directory Based Edge Blocking to Reject Messages Sent to Invalid Recipients Describes the Directory Based Edge Blocking feature which lets you reject messages for invalid recipients at the service network perimeter.
View or Edit Managed Domains in EOP describes how to manage domains that are associated with your EOP service.
If you add subdomains to your organization, your EOP service can help you manage these too. Learn more about subdomains at Enable Mail Flow for Subdomains in Exchange Online.
Configure mail flow using connectors in Office 365 introduces connectors and shows how you can use them to customize mail routing. Scenarios include ensuring secure communication with a partner organization and setting up a smart host.
To ensure that junk email is routed correctly to each user's junk-email folder, you must perform a couple configuration steps. These are detailed in Ensure that spam is routed to each user's Junk Email folder. If you do not want to move messages to each user's junk-email folder, you may choose another action by editing your content filter policies in the Exchange admin center. For more information, see Configure your spam filter policies.
Verify mail flow
To verify that your EOP setup, including your connector configuration, is working correctly, see the "How do you know this task worked?" section in Set up your EOP service.
Test Mail Flow with the Remote Connectivity Analyzer provides instructions for testing that your mail flow is set up correctly.