Should You Use the @ Symbol When Blocking Domains in Microsoft 365 Defender?
Question details
The user needs to know the correct syntax regarding the use of the '@' symbol when adding block entries in Microsoft 365 Defender.
- Product
- Microsoft 365 Defender
- Device & OS
- not provided
- Scenario
- Configuring security policies to block unwanted or malicious domains and email senders.
- Observed behavior
- The user is unsure whether to include the '@' symbol to successfully block an entire domain versus a specific email sender.
Ensure you have the necessary administrator permissions (such as Security Administrator) in your Microsoft 365 tenant to modify the Tenant Allow/Block Lists.
Apply Correct Formatting for Domains and Email Addresses
Use specific syntax depending on whether you are blocking an entire domain or an individual email address.
When managing the Tenant Allow/Block Lists in Microsoft 365 Defender, the system distinguishes between full domains and individual senders based on the syntax you provide.
Using the incorrect format can result in policy errors or the block rule failing to trigger on incoming malicious emails.
To block all incoming mail from a specific domain, enter only the domain name without the @ symbol. For example, type 'example.com' to block the entire domain.
To block an individual sender, include the @ symbol along with the full email address. For example, type 'user@example.com'.
Log in to the Microsoft 365 Defender portal, navigate to 'Email & collaboration' in the left menu, and select 'Policies & rules'.
Click on 'Threat policies', select 'Tenant Allow/Block Lists', navigate to the 'Domains & addresses' tab, and click 'Block' to add your properly formatted entries.
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Frequently Asked Questions
What happens if I accidentally use the @ symbol when trying to block a whole domain?
Microsoft 365 Defender will either reject the entry as an invalid email format during setup, or the rule will fail to block the intended domain because it expects a valid username before the @ symbol.
Can I use wildcards to block subdomains in Microsoft 365 Defender?
Yes, you can use an asterisk to block subdomains (e.g., '*.example.com'). However, just like with standard domains, you must not include the @ symbol when configuring wildcard domain blocks.
How long does it take for a new domain block to take effect?
Once you successfully add a domain or email address to the Tenant Allow/Block List with the correct syntax, it typically takes up to 5 minutes for the block to become fully active across your organization.
