My outgoing email not working

Hello @Aejaz,

Based on the NDR you've shared, this issue is typically not related to Outlook itself. The error indicates that Exchange Online has blocked the mailbox from sending outbound email, usually because Microsoft's anti-spam systems detected suspicious sending activity, excessive email volume, or signs that the account may have been compromised.

If you are the Microsoft 365 administrator, I you may want to follow these steps:

Step 1: Check whether the account is blocked

• Sign in to the Microsoft 365 Defender portal.
• Go to Email & collaboration.
• Select Review.
• Open Restricted entities.

• Search for the affected email address.

If the account appears in the list, it has been restricted from sending mail.

Step 2: Secure the account

Before removing the restriction:

• Reset the user's password.
• Sign out the user from all sessions (if applicable).
• Enable MFA if it is not already enabled.
• Review recent sign-in activity for unusual locations or devices.
• Check the Sent Items folder for messages the user did not send.

Step 3: Remove the restriction

• Return to Microsoft 365 Defender > Review > Restricted entities.
• Select the affected mailbox.
• Click Unblock.
• Confirm the action.

Once unblocked, it may take some time for outbound mail flow to be restored.

Step 4: Test mail flow

After the restriction is removed:

• Send a test email to an external mailbox (for example, Gmail or Outlook.com).
• Verify whether the message is delivered successfully.
• If the NDR persists, review message traces and account activity for further signs of abuse.

If you are not the administrator, please contact your Microsoft 365 admin and provide them with the complete NDR message. They will need to verify whether the sender has been placed on the Restricted entities list and remove the restriction if appropriate.

For your reference:

• NDR error code "550 5.1.8 Access denied, bad outbound sender" | Microsoft Support
• Remove blocked users from the Restricted entities page - Microsoft Defender for Office 365

Please let me know the result after all the steps, and I'll be happy to assist further if needed.