How to fix "This message couldn't be delivered because the sending email address was not recognized as a valid sender" error

Question

When trying to send an email, I receive an error message stating "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." The documentation suggests removing the user from restricted entities, but the user is not available in restricted entities in Defender. How can I resolve this issue?

Answer 1

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.

Answer 2

Hi @Rudraraj Mer,

Welcome to post our Q&A forum!

The error message you received indicates that your email address is suspected of sending spam and is no longer allowed to send messages outside of your organization.

If the user is not available in the Restricted entities page, it’s possible that the user is not blocked due to spamming. In this case, you may want to try the following:

• Check if the user is blocked due to other reasons such as suspicious activity or security concerns.
• Verify that the email address is correct and that there are no typos.
• If you received this error when sending a message using Outlook or another email app, try using Outlook.com to send the message instead.
• If you’re still having trouble, you can try contacting your email administrator for further assistance. They can help you determine if your account has been added to the action center and remove it if necessary.

https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/outbound-spam-restore-restricted-users?view=o365-worldwide

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