Send & Receive Issues

How to Fix Error 550 5.1.8 Access Denied Bad Outbound Sender in Microsoft 365

Olivia Miller Sep 28, 2026 869 views

Question details

The user is unable to send emails and receives a Non-Delivery Report (NDR) containing the error code 550 5.1.8, indicating the account has been blocked as a bad outbound sender.

How to Fix Error 550 5.1.8 Access Denied Bad Outbound Sender

Product: Microsoft Exchange Online
Device & OS: not provided
Scenario: Attempting to send an outbound email to any recipient through a Microsoft 365 email account.
Observed behavior: The outgoing message is blocked by Exchange Online, and the system immediately returns an NDR with the message "550 5.1.8 Access denied, bad outbound sender."

Before you start

Do not delete the Non-Delivery Report (NDR) email you received. It contains crucial diagnostic headers and error codes that your IT administrator will need to unblock your account.

Solution 1Recommended

Contact Your Microsoft 365 Administrator to Unblock the Account

This error means the account was restricted by Microsoft for sending suspected spam. Only a global administrator can lift this restriction via the admin portal.

When an account exceeds sending limits or exhibits suspicious activity, Microsoft 365 automatically blocks it from sending outbound messages to protect the server's reputation. End-users cannot bypass this block manually.

Save the Non-Delivery Report (NDR)

Open the bounce-back email you received and save the full error message, including the specific error code and the message headers.

Contact IT Support

Forward or provide the NDR details to your organization's Microsoft 365 Global Administrator or your dedicated IT support department.

Unblock via Microsoft 365 Defender (For Admins)

The administrator must log in to the Microsoft 365 Defender portal, navigate to Email & Collaboration > Review > Restricted entities, select the affected user account, and choose the option to unblock it.

Contact Your Microsoft 365 Administrator to Unblock the Account

Admin Permissions Required: End-users do not have the necessary permissions to remove themselves from the restricted senders list. You must reach out to the person who manages your company or school's email system.

Solution 2

Submit a Microsoft Service Request (For Administrators)

If the administrator cannot unblock the account through the Defender portal, they must contact Microsoft backend support by opening a service request.

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Frequently Asked Questions

Why did I receive the 550 5.1.8 Access Denied error?

This error occurs when your email account is suspected of sending spam or malicious emails. To protect the server's reputation, Microsoft 365 automatically places the account on a restricted list, blocking it from sending any further outbound messages.

Can I fix the 550 5.1.8 bad outbound sender error myself?

No, if you are a standard end-user, you cannot fix this yourself. You must contact your organization's Microsoft 365 global administrator, as they hold the necessary backend permissions to unblock your account in the Microsoft 365 Defender portal.

How long does it take to send emails again after being unblocked?

Once your administrator removes your account from the Restricted entities page in Microsoft 365 Defender, it typically takes a few hours—and sometimes up to 24 hours—for the restrictions to be fully lifted across all Microsoft servers.

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