My Email Keeps Bouncing

Question

My Email Keeps Bouncing

Michalis Fotis 20

How do I fix it as the message it keeps sending back is

Delivery has failed to these recipients or groups:

[<PII removed>.co.uk] 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: <PII removed>.PROD.OUTLOOK.COM

<PII removed> Remote server returned '550 5.1.8 Access denied, bad outbound sender AS(42004)'

Original message headers:

Received: from <PII removed>.PROD.OUTLOOK.COM
 ([fe80::99b3:c5e2:c972:1979]) by <PII removed>.PROD.OUTLOOK.COM
 ([fe80::99b3:c5e2:c972:1979%6]) with mapi id <PII removed>; Sun, 7 Jun 2026
 20:51:02 +0000
MIME-Version: 1.0
Content-Type: text/plain
Date: Sun, 7 Jun 2026 20:51:02 +0000
Message-ID:
       <<PII removed>.PROD.OUTLOOK.COM>
Subject: Quick Question For Story Of Home

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Answer 1

Anonymous

HI @Michalis Fotis

Please note that our forum is a public platform, and we will modify your question to hide your personal information in the description. Kindly ensure that you hide any personal or organizational information the next time you post an error or other details to protect personal data.

The error message 550 5.1.8 Access denied, bad outbound sender AS(42004) indicates that the sending account has been restricted by Exchange Online and is no longer permitted to send outbound email, typically due to suspicious or spam-like activity.

If you are a Microsoft 365 administrator, please check whether the affected account appears under Microsoft 365 Defender > Email & Collaboration > Review > Restricted Entities. If the account is listed there, it can be reviewed and unblocked.

Before removing the restriction, Microsoft recommends verifying that the account has not been compromised and is not sending unsolicited or suspicious email. Once the account is unblocked, the restriction is typically removed within approximately one hour.

If you are a standard user and do not have administrative access to Microsoft 365, please contact your IT administrator and provide them with the NDR message. They can review the account status and determine whether it has been added to the Restricted Entities list.

If you’re unsure who your IT administrator is, see How do I find my Microsoft 365 admin? - Microsoft Support

Reference: NDR error code "550 5.1.8 Access denied, bad outbound sender" - Microsoft Support

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Michalis Fotis 20 Reputation points

2026-06-08T15:58:11.6966667+00:00

Please give me a step by step guide on what to do next as I can't see what you have stated in the comment.

Thank you
Anonymous

2026-06-09T00:27:44.7566667+00:00

Hi @Michalis Fotis
Could you let me know whether you’re an admin or a standard user? That way, I can tailor my suggestions appropriately.
Michalis Fotis 20 Reputation points

2026-06-09T07:37:59.8666667+00:00

Please give me a step by step on what to do next as this isn't working.

It would be ideal if I could get on the phone with someone from Microsoft and speak to them directly.
Anonymous

2026-06-10T04:30:48.8266667+00:00

Hi @Michalis Fotis
If you are an admin, please follow these steps:

Go to Microsoft 365 Defender > Email & Collaboration > Review > Restricted Entities.

From there, review and unblock the account marked as spam, provided you have verified that it has not been compromised.

Please note that only administrators have the ability to perform this action. While normal users may access the portal, they do not have the necessary permissions to change these settings.

Just a quick note that this forum is a peer-to-peer community, so while members can share advice and experiences, it doesn’t offer phone or live support.

You can find a customer service phone number in your country here Customer service phone numbers - Microsoft Support

If you have an active Microsoft 365 subscription and hold the Global Admin role, you can submit a support request directly through the Microsoft 365 admin center. After you submit the request, a Microsoft Support agent will call you back.

You can read more at: Get support - Microsoft 365 admin | Microsoft Learn
Anonymous

2026-06-11T03:47:33.1366667+00:00

Hi @Michalis Fotis

May I know if you have got any chance to check my answer? If you need further assistance, please feel free to let me know. Thank you!