Send & Receive Issues

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

Elise Williams Oct 1, 2026 868 views

Question details

The user is unable to send emails and receives a 550 5.1.8 Bad Outbound Sender error, indicating the account is restricted for suspected spam activity.

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

Product: Microsoft 365 Exchange
Device & OS: not provided
Scenario: Attempting to send an outbound email from a Microsoft 365 Exchange account.
Observed behavior: Email delivery fails with an error message stating the sender is not recognized as valid or has been blocked by outbound spam protection.

Before you start

Before proceeding, ensure you disconnect your device from the network to prevent further unauthorized activity and immediately notify your IT administrator that your account may be compromised.

Solution 1Recommended

Secure the Account and Remove Malicious Configurations

Take immediate action to lock out unauthorized users and clean up any malicious settings before requesting an unblock.

When an account is flagged with a 550 5.1.8 error, it usually means a third party has gained access and is sending spam. Securing the account is the required first step before an administrator will restore email functionality.

Reset your password

Log in to your Microsoft 365 account portal or contact your IT department to change your password immediately. Use a strong, unique password to prevent further unauthorized access.

Scan your device for malware

Run a full system scan using a reputable and updated antivirus program to ensure there is no malware or keyloggers on your computer.

Inspect mailbox rules and forwarding

Open Outlook or Outlook on the web, go to Settings, and check your 'Rules' and 'Forwarding' sections. Delete any unfamiliar rules that may be hiding emails or forwarding them to external addresses.

Check delegate access

Verify that no unauthorized users have been granted delegate access to your mailbox by checking your account's delegation settings in Outlook.

Secure the Account and Remove Malicious Configurations

Important: Completing these steps does not automatically unblock your account. You must still have your administrator remove the block in the backend.

Solution 2

Administrator Action to Unblock the User

If you are a Microsoft 365 admin, use the Microsoft 365 Defender portal to review and remove the outbound spam restriction.

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Frequently Asked Questions

Why did I receive a 550 5.1.8 error when trying to send an email?

This error occurs when Microsoft 365's outbound spam protection system suspects your account is sending bulk spam or has been compromised. To protect the network's reputation, the system automatically restricts your ability to send outbound emails.

Can I unblock my account myself if I get the 550 5.1.8 error?

Typically, end users cannot lift an outbound spam restriction on their own. You must contact your organization's Microsoft 365 IT administrator, who can investigate the issue and unblock your account via the Microsoft 365 Defender portal.

How long does it take to send emails again after the admin unblocks my account?

Once your administrator successfully removes the restriction in the Microsoft 365 Defender portal, it usually takes between 1 to 24 hours for the system changes to fully propagate and restore your outbound email delivery.

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