Why couldn't my message be delivered?

Question

I received the following error message when trying to send an 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 administrator for assistance and provide them with this error message."

Could you explain what causes this error and why my email address is being rejected as an invalid sender? What steps can be taken to restore the sender's reputation and allow emails to be delivered successfully?

Answer 1

Good day Ward, 

Based on the error message, it appears that Microsoft 365 has temporarily restricted your mailbox from sending emails. This can happen when the account exceeds sending limits or when unusual outbound email activity is detected, which may indicate spam or a possible account compromise. 

This doesn't necessarily mean you sent spam intentionally. The restriction is a security measure designed to protect your account and other recipients. 

I recommend contacting your IT administrator and sharing the full error message. In case you do not know who is your IT admin, kindly refer to this article: How do I find my Microsoft 365 admin? - Microsoft Support  

Before removing the restriction, they should verify that the account has not been compromised by reviewing any suspicious activity, resetting credentials if needed and removing any unauthorized inbox rules or access. 

Your administrator can then go to Microsoft 365 Defender > Email & Collaboration > Review > Restricted Entities, locate the affected mailbox and unblock it after confirming the account is secure.  

Alternatively, they can also use Exchange Online PowerShell to manage restricted senders: 

Run Get-BlockedSenderAddress to view restricted senders. 

Run Remove-BlockedSenderAddress to remove the sender from the restricted list. 

I hope this helps explain the issue and the steps needed to restore normal email sending. If you have any questions or need further support, please don’t hesitate to reach out at any time. 

Thank you for your patience and understanding. 

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Answer 2

This error means the sending account is no longer trusted to send outbound mail. In Microsoft 365 and Exchange Online, this commonly happens when the account is suspected of sending spam or when it has exceeded Exchange Online sending limits. The resulting non-delivery report is 550 5.1.8 Access denied, bad outbound sender.

Common causes supported by the documentation include:

1. The account was flagged for spam-like or unusual outbound activity
   • Microsoft 365 can restrict a sender if the account appears compromised or is sending messages that resemble spam.
2. The account exceeded Exchange Online sending limits
   • This specific NDR is generated if a user tries to send mail after the account exceeds Exchange Online sending limits.
3. The sender or domain has deliverability problems
   • Legitimate mail can also be treated suspiciously if email authentication fails:
     • SPF
     • DKIM
     • DMARC
   • Poor sender reputation, blocklist history, missing reverse DNS, or spam-triggering content can also contribute to mail being flagged.

To restore sending ability:

• If the sender is a user:
  1. Check whether the account is compromised.
  2. Notify the Microsoft 365 email administrator.
• If the sender is an administrator:
  1. Secure the account first if compromise is suspected.
  2. In the Microsoft 365 Defender portal, go to Restricted entities and unblock the user account.
  3. After unblocking, restrictions are usually removed within one hour.

To help restore and protect sender reputation:

1. Fix email authentication
   • Verify SPF, DKIM, and DMARC are correctly configured for the sending domain.
2. Review sender reputation factors
   • Check whether the sending IP is on non-Microsoft blocklists.
   • Verify the sending IP has a valid reverse DNS (PTR) record.
   • If relevant, review sender reputation in Microsoft SNDS.
3. Review message content
   • Avoid characteristics commonly associated with spam, such as excessive links, URL shorteners, embedded scripts, image-only messages, or content that matches enabled ASF settings.
4. Reduce future risk of restriction
   • Investigate whether the account was used for bulk or high-volume sending.
   • Follow Microsoft 365 security best practices to prevent compromise and repeated blocking.

If the environment is Outlook.com rather than Microsoft 365 business mail, blocked or blacklist-related delivery failures can also occur because the recipient system considers the sending account or mail server a spam source. In that case, the recipient provider may need to remove the block.

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• NDR error code "550 5.1.8 Access denied, bad outbound sender"
• Frequently asked questions - Anti-spam protection
• Sender Support in Outlook.com
• Sent email in Outlook.com comes back "delivery failed"