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How to Fix SMTP Error 451 4.4.4 for Undelivered Email in Microsoft 365
Question details
The user needs to resolve an email delivery failure resulting in SMTP error 451 4.4.4.
- Product
- Microsoft 365 / Exchange Online
- Device & OS
- not provided
- Scenario
- Sending an email to a recipient domain hosted on Microsoft 365.
- Observed behavior
- The email bounces back or fails to deliver, returning an SMTP error code 451 4.4.4, indicating the recipient lacks an active mailbox license.
Ensure you have administrator access to the Microsoft 365 admin center, as resolving this issue requires verifying recipient mailbox licenses and domain configurations.
Verify Microsoft 365 Licenses and Mailbox Settings
Check if the recipient has a valid, active Exchange Online license and verify domain mail flow settings.
SMTP error 451 4.4.4 usually happens when a tenant's domain is configured in Microsoft 365, but the specific recipient address does not have a fully licensed Exchange mailbox.
Go to admin.microsoft.com and sign in with your global administrator or Exchange administrator credentials.
Navigate to 'Users' > 'Active users' on the left sidebar and select the recipient experiencing the delivery issue.
Click on the 'Licenses and apps' tab in the user pane to ensure the user has an active Exchange Online license assigned.
Go to 'Settings' > 'Domains' to verify that the recipient's domain is marked as 'Healthy' and properly configured for mail flow.
Open a Support Ticket with Microsoft
If all licenses and domain settings are correct but the error persists, contact Microsoft support for backend resolution.
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Frequently Asked Questions
What does SMTP Error 451 4.4.4 mean?
This error generally indicates a temporary server error. In the context of Microsoft 365, it typically means the email could not be delivered because the recipient's domain is hosted in Microsoft 365, but the specific user lacks an active Exchange Online mailbox or valid mail-enabled subscription.
Can the sender fix SMTP Error 451 4.4.4?
No, this is a recipient-side issue. The sender should contact the recipient through an alternative method (like a phone call) and advise them to have their IT administrator check their Microsoft 365 mailbox licensing and domain settings.
How long does it take to fix the mail flow after assigning a license?
Once a Microsoft 365 administrator assigns an active Exchange Online license to the recipient, it can take anywhere from a few minutes up to 24 hours for the mailbox to fully provision and begin successfully receiving external emails.
