How to fix the mail deliverability issue on my exchange account?

Anonymous
2025-11-03T00:28:40.6866667+00:00

I am no longer able to send any email to anyone. I can receive email, but I can't send any. How could I fix it? Here is the error I get for every attempt when trying to send an email.

Delivery has failed to these recipients or groups:

[Recipient Email] This message couldn't be delivered because the sending email address was not recognized as a valid sender. The most common reason for this error is that the email address is, or was, suspected of sending spam. Contact the organization's email admin for help and give them this error message.

Diagnostic information for administrators:

Generating server: OUTLOOK.COM

[Recipient Email] Remote server returned '550 5.1.8 Access denied, bad outbound sender AS(42004)'

Original message headers:

Received: from OUTLOOK.COM
by [PII removed]
MIME-Version: 1.0
Content-Type: text/plain
Date:
Message-ID:
Subject: checking
Exchange | Exchange Server | Management
Exchange | Exchange Server | Management

The administration and maintenance of Microsoft Exchange Server to ensure secure, reliable, and efficient email and collaboration services across an organization.

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Answer accepted by question author
Anonymous
2025-11-03T03:14:40.1266667+00:00

Hi @Ima M

Thank you for posting your question in the Q&A Forum and sharing the details of the error. I completely understand how frustrating it can be when important emails don’t go through. 

The error message you received, 550 5.1.8 Access denied, bad outbound sender is a security measure from the Microsoft 365/Exchange Online system. It indicates that your account has been temporarily restricted from sending emails due to detected unusual activity. 

This usually happens if the system suspects spam-like behavior or believes the account may have been compromised. However, only your organization's Email Administrator or IT team has the permissions to remove the block. 

If you’re the user: 

  • Determine whether your account has been compromised (e.g., check your Sent Items for unfamiliar emails or unusual Inbox Rules). If you suspect your account has been compromised, change your password immediately. 
  • After securing your account, contact your email administrator for further assistance with unblocking your account. 

If you’re an email administrator, follow these steps: 

To help prevent future account compromises and security incidents, Microsoft strongly recommends following security best practices for your organization's setup. Here’s a detailed guide: Microsoft 365 for business security best practices 

Please let us know once you’ve completed these steps, and we’ll be happy to assist further if needed. 

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".    

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread. 

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