Safelist a domain in Exchange Online

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This article provides two methods to safelist, or whitelist, a domain in Exchange® Online for Microsoft 365®. Safelisting a domain prevents messages sent from that domain from being filtered as spam by the Exchange Online spam filter. Instead, Exchange Online sends the message directly to the user's inbox. When you safelist a domain, ensure that your users remain vigilant against the risk of spoofing.

Prerequisites

Applies to: Administrator
Difficulty: Moderate
Time Needed: Approximately 25 minutes
Tools Needed: Global administrator access for Microsoft 365

For more information about prerequisite terminology, see Cloud Office support terminology.

Safelist a domain by using the allowed senders list

Log in to your Office 365 Control Panel.
From the left-hand menu, select Office 365 Admin Center.
Select Show all from the left-hand menu and then select Exchange under the Admin centers section.
Select Protection from the left menu and then click the Spam Filter tab.
Select your Default spam filter policy (or the policy with the Relative priority set to Lowest) and then select the pencil icon to edit the policy.
Select Allow Lists.
Select the + icon below the Domain allow list section.
Enter the domain you want to safelist in the text box. Separate multiple domains by using a semicolon or use a new line.
Select OK and then select Save.

Any emails sent from the domains in your Domain allow list are now delivered to your users' inbox successfully. This delivery includes mail with spoofed sender addresses. The next section shows you how to safelist a domain while reducing the likelihood of receiving spoofed messages.

Safelist a domain by using mail flow rules

When you use mail flow rules to bypass spam filtering, Exchange Online can perform some authentication checks for the domain you want to bypass. This more complicated method reduces but does not eliminate the risk of allowing unauthenticated senders to deliver spoofed mail.

Log in to your Office 365 Control Panel.
From the left-hand menu, select Office 365 Admin Center.
Select Show all from the left menu and then select Exchange under the Admin centers section.
Select Mail Flow from the left menu and then select the Rules tab.
Select the + icon and then choose Bypass spam filtering from the drop-down menu.
Give the rule a descriptive name such as Bypass spam filtering for domain.com.
Under the Apply this rule if section in the first drop-down menu, perform the following tasks:

a. Select The sender.

b. Select domain is.

c. When prompted, type the domain you want to safelist into the text box.

d. Select the + icon.

e. Click OK.
Select Add Condition and perform the following tasks:

a. From the new drop-down menu, select The sender.

b. Select is external/internal.

c. When prompted, select Outside the organization from the drop-down menu.

d. Click OK.
Select Add Condition and perform the following tasks:

a. From the new drop-down menu, select A message header.

b. Select includes any of these words.
Select the Enter text… hyperlink on the right side and perform the following tasks:

a. In the Specify Header Name field, enter Authentication-Results.

b. Select OK.
Select the Enter words hyperlink on the right side, and in the text box, enter dmarc=pass.
Then perform the following tasks:

a. Select +.

b. Type dmarc=bestguesspass.

c. Select +.

d. Click OK.
Select Add Action from the new drop-down menu and perform the following tasks:

a. Select Modify the message properties.

b. Select set a message header.
Select the first Enter text… hyperlink on the right and perform the following tasks:

a. Enter X-ETR into the message header text box.

b. Select OK.
Select the second Enter text hyperlink on the right and perform the following tasks:

a. Enter Bypass spam filtering for authenticated sender domain.com.

b. Select OK.
Select Save.

Your rule now bypasses spam filtering for your specified domain while allowing Exchange Online to perform a Domain-based Message Authentication, Reporting, and Conformance (DMARC) check. It also follows Microsoft best practices by modifying the message headers to include details about bypassing the spam filter, which provides more information for administrators when troubleshooting issues.

Note: If you notice any issues with mail delivery, we recommend that you turn off the rule by unchecking it in the rules list. Then use the steps under the preceding Safelist a domain by using the allowed senders list section.

Additional Resources

To learn more about creating safe senders lists in Microsoft 365, see Create safe sender lists in EOP.

Updated 5 months ago