Can't send or reply to messages

Question

When i try to reply or send a message this pops up. 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 1

Hi @Roshunna Younger,

Thank you for reaching out regarding the issue you’re experiencing when trying to send or reply to emails.

Based on the error message you’re seeing, it appears that your sending email address is currently restricted from sending outbound messages. This typically occurs when the system detects activity that may be associated with spam or unusual sending behavior.

With that in mind, here are the recommended next steps:

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1/ Check if your account might be compromised

Blocks are often triggered by compromised accounts that start sending spam. To be safe, you can:

• Change your password immediately (use a strong, unique password) or ask your IT administrator to reset password for your account.
• Ensure multi‑factor authentication (MFA) is enabled and working (e.g., Microsoft Authenticator).
• Review sign‑in activity to see if there's any unfamiliar activities by going to My Account page > Recent activity

2/ Try sending in a different way (just as a test)

Try sending from Outlook desktop / mobile or from another browser / InPrivate window.

If it fails everywhere with the same message, that confirms this is a server-side block, not a browser issue.

3/ Collect the details to send to your admin

Please contact your IT admin and share the full error message so they can check whether your account is listed as a restricted sender and request removal if applicable. For reference, you can share the guidance in this article with your IT administrator: Remove blocked users from the Restricted entities page - Microsoft Defender for Office 365 | Microsoft Learn.

After your admin completes these steps, please wait approximately 1–2 hours and then try sending an email again to confirm whether the issue still persists.

4/ Contact Microsoft support

If the issue persists after applying these steps, the best next step would be to have your IT administrator open a support ticket with Microsoft via the admin center. A technical support engineer can then initiate a remote session, review backend settings, and run diagnostics. If needed, they can escalate the case to specialized engineering teams who are well-equipped to perform advanced troubleshooting and provide targeted solutions.

As moderators, our primary goal is to provide helpful guidance and support through general troubleshooting steps. While we don’t have access to internal systems or administrative tools required to resolve account-specific or backend issues, we truly appreciate your understanding of these limitations. We genuinely hope the information we share helps point you in the right direction, and we're always here to assist as much as we can within our scope.

For guidance on how to request support in the admin center, your IT admin can please refer to: Get support - Microsoft 365 admin. 

I hope this information is useful. Thank you for taking the time to raise this question. If you need any further clarification or assistance, please feel free to leave a comment under this post.

If you find my post useful, kindly consider upvoting it. Doing so can assist others in the community who may have similar questions in finding solutions more quickly.

Thank you for your contributions to the forum.

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