Resolving – 550 5.1.8 Access denied, bad outbound sender

You’ve encountered the error “Remote Server returned ‘550 5.1.8 Access denied, bad outbound sender AS(42004)” in Exchange Online. This error typically occurs when a sender exceeds outbound limits or sends suspicious emails, such as spam. To resolve this issue, follow these steps:

Step 1: Identify the Affected User

1. Log in to your Exchange Online Admin Center: Go to https://admin.microsoft.com and sign in with your administrator credentials.
2. Access Message Trace: In the Admin Center, navigate to “Mail Flow” and select “Message Trace.” Here, you can search for messages sent by the affected user by specifying their email address or other relevant criteria.
3. Review Suspicious Activity: Look for any suspicious email activity from the user, such as a high volume of sent emails or unusual content. This may help identify the cause of the error.

Step 2: Remove the User from Restricted Entities

1. Access Security Center: In the Admin Center, navigate to “Security” and select “Review” from the left menu. Then choose “Restricted entities.”
2. Find the User: Locate and select the user causing the issue in the list of restricted entities.
3. Remove the User: Click on “Remove” or take appropriate action to release the restriction on the user.
   This will take one hour to take effect. Then, the system will allow the user to send an email.

Step 3: Prevent Future Occurrences

1. Educate Users: Train your users on email best practices, including avoiding suspicious attachments, not sending spam, and adhering to outbound limits.
2. Implement Monitoring: Set up monitoring and alerts for suspicious email activity to catch and address issues early.
3. Adjust Outbound Policies: Consider adjusting your outbound email policies, if necessary, to prevent users from hitting sending limits.

Step 4: Monitor and Test

1. Monitor the Situation: Keep an eye on the affected user’s email activity to ensure the error doesn’t recur.
2. Test Outbound Emails: Have the user send a test email to confirm that the issue has been resolved.

By following these steps, you can effectively resolve the Exchange Online error “550 5.1.8 Access denied, bad outbound sender AS(42004)” and prevent it from happening again in the future. Remember to stay vigilant and educate your users to maintain a secure email environment.