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How to Block All Emails from the .jp Domain in Microsoft 365
Question details
The user wants to block incoming emails from all domains ending in the .jp country-code suffix.
- Product
- Microsoft 365
- Device & OS
- not provided
- Scenario
- Configuring email security policies in the admin center to block spam or unwanted emails originating from a specific country-code top-level domain.
- Observed behavior
- The Microsoft 365 admin center's blocked-domain field rejects incomplete domains (like just ".jp") and requires fully qualified domain names (like "example.jp").
Ensure you have global administrator or Exchange administrator privileges in Microsoft 365 to create and manage mail flow rules in the Exchange admin center.
Create a Mail Flow Rule in the Exchange Admin Center
Since the standard blocked-domain list requires full domain names, creating a mail flow rule allows you to block emails based on domain suffixes like .jp using pattern matching.
Mail flow rules (also known as transport rules) in Exchange Online provide the flexibility to inspect messages for specific conditions, such as sender address patterns, and take action before the email reaches the user's inbox.
Log in to the Microsoft 365 admin center, click on 'Show all' in the left navigation pane, and navigate to 'Admin centers' > 'Exchange'.
In the Exchange admin center, expand the 'Mail flow' menu on the left side and select 'Rules'.
Click the '+' (Add) icon and select 'Create a new rule'. Give the rule a clear name, such as 'Block all .jp domains'. If necessary, click 'More options' at the bottom of the window to reveal all available conditions and actions.
Under the 'Apply this rule if...' section, select 'The sender's address includes any of these words' (or use 'The sender's domain matches any of these text patterns' if available). Enter '.jp' and click the '+' icon to add it to the list, then click 'OK'.
Under the 'Do the following...' section, select 'Block the message', and then choose either 'reject the message and include an explanation' or 'delete the message without notifying anyone' depending on your organization's policy.
Review the configurations and click 'Save'. The new rule will take effect shortly and automatically block any incoming email originating from a domain ending in .jp.
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Frequently Asked Questions
Why can't I just type .jp into the blocked domains list in Microsoft 365?
The standard anti-spam policies in the Microsoft 365 admin center are designed to accept fully qualified domain names (like example.com or domain.jp). They do not support wildcard or suffix-only entries (like just .jp), which is why a mail flow rule is necessary to match the pattern.
Can I block multiple country-code domains using the same mail flow rule?
Yes. When setting up the condition in the Exchange admin center, you can add multiple text strings or patterns. Simply add .ru, .cn, or any other top-level domain suffixes you wish to block alongside .jp in the same rule condition.
Will this rule block internal emails if a user's address contains .jp?
If properly configured, the rule applies to incoming emails based on the sender's address matching the string. To prevent accidental blocking of internal communication, you can add an exception to the rule (under 'Except if...') specifying that the sender is located 'Inside the organization'.
Does this require a premium Microsoft 365 subscription?
No. The Exchange admin center and mail flow rules are standard features available in all Microsoft 365 business and enterprise plans that include Exchange Online.
