Use Directory-Based Edge Blocking to reject messages sent to invalid recipients in Exchange Online
Directory-Based Edge Blocking (DBEB) lets you reject messages for invalid recipients at the service network perimeter in Microsoft 365 organizations with Exchange Online mailboxes and in standalone Exchange Online Protection (EOP) organizations without Exchange Online mailboxes. DBEB lets admins add mail-enabled recipients to Microsoft 365 or Office 365 and block all messages sent to email addresses that aren't present in Microsoft 365 or Office 365.
If a message is sent to a valid email address in Microsoft 365 or Office 365, the message continues through the rest of the service filtering layers: anti-malware, anti-spam, and mail flow rules (also known as transport rules). If the address doesn't exist, the service blocks the message before filtering even occurs, and a non-delivery report (also known as an NDR or bounce message) is returned to the sender. The NDR looks like this: 550 5.4.1 Recipient address rejected: Access denied.
If all recipients for your domain are in Exchange Online, DBEB is already in effect, and you don't need to do anything. If you're migrating from another email system to Exchange Online, you can use the procedure in this topic to enable DBEB for the domain before the migration.
Note
- In hybrid environments, in order for DBEB to work, the MX record for the domain must point to Microsoft 365 or Office 365 so that email for the domain is routed to Microsoft 365 or Office 365 first.
- There are additional considerations when using DBEB with on-premises mail-enabled public folders. For more information about DBEB and mail-enabled public folders, see Office 365 Directory Based Edge Blocking support for on-premises Mail Enabled Public Folders.
What do you need to know before you begin?
Estimated time to complete: 5 to 10 minutes
To open the Exchange admin center (EAC), see Exchange admin center in Exchange Online.
For information about keyboard shortcuts that may apply to the procedures in this topic, see Keyboard shortcuts for the Exchange admin center.
Tip
Having problems? Ask for help in the Exchange forums. Visit the forums at: Exchange Online, or Exchange Online Protection.
Configure DBEB
This section describes the procedure to configure DBEB for both the New Exchange admin center (EAC) and Classic EAC.
For New EAC
Verify that your accepted domain in Exchange Online is set to Internal relay:
a. Navigate to Mail flow > Accepted domains. The Accepted domains screen appears.
b. Select an accepted domain and click it. The accepted domain's details screen appears.
c. Ensure that the domain type is set to Internal relay. If it's set to Authoritative, change it to Internal relay.
d. Click Save.
Add users to Microsoft 365 or Office 365. For example:
- Directory synchronization: Add valid users to Office 365 by synchronizing from your on-premises Active Directory environment to Microsoft Entra ID in the cloud. For more information about how to set up directory synchronization, see Use directory synchronization to manage mail users.
- Add users via PowerShell or the EAC: For more information about how to do this task, see Manage mail users in Exchange Online (and EOP).
Set your accepted domain in Exchange Online to Authoritative:
a. Navigate to Mail flow > Accepted domains. The Accepted domains screen appears.
b. Select an accepted domain and click it. The accepted domain's details screen appears.
c. Ensure that the domain type is set to Authoritative. If it's set to Internal relay, change it to Authoritative.
d. Click Save.
For Classic EAC
Verify that your accepted domain in Exchange Online is set to Internal relay:
a. Navigate to Mail flow > Accepted domains.
b. Select an accepted domain and click Edit.
c. Ensure that the domain type is set to Internal relay. If it's set to Authoritative, change it to Internal relay.
d. Click Save.
Add users to Microsoft 365 or Office 365. For example:
- Directory synchronization: Add valid users to Office 365 by synchronizing from your on-premises Active Directory environment to Microsoft Entra ID in the cloud. For more information about how to set up directory synchronization, see Use directory synchronization to manage mail users.
- Add users via PowerShell or the EAC: For more information about how to do this task, see Manage mail users in Exchange Online (and EOP).
Set your accepted domain in Exchange Online to Authoritative:
a. Navigate to Mail flow > Accepted domains.
b. Select an accepted domain and click Edit.
c. Set the domain type to Authoritative.
d. Click Save.
Choose Save to save your changes, and confirm that you want to enable DBEB.
Note
- Dynamic distribution groups created in Exchange on-premises don't sync to Exchange Online and are therefore blocked by DBEB. As a workaround in hybrid environments, you can create a mail contact with the same external email address of the blocked dynamic distribution group in Exchange Online.
- Until all of your valid recipients have been added to Exchange Online and replicated through the system, you should leave the accepted domain configured as Internal relay. Once the domain type has been changed to Authoritative, DBEB is designed to allow any SMTP address that has been added to the service. There might be infrequent instances where recipient addresses that don't exist in your Microsoft 365 or Office 365 organization are allowed to relay through the service.