PLease help me fix this error which i have received and has disabled my mail from being delivered. I have logged into Defender but am unable to understand if i have resolved the problem.

Question

PLease help me fix this error which i have received and has disabled my mail from being delivered. I have logged into Defender but am unable to understand if i have resolved the problem.

Sam Laite 0

Diagnostic information for administrators:

Generating server: AS8PR01MB7672.eurprd01.prod.exchangelabs.com

******@gmail.com Remote server returned '550 5.1.8 Access denied, bad outbound sender AS(42004)'

Original message headers:

Received: from AS8PR01MB7672.eurprd01.prod.exchangelabs.com
 ([fe80::c1b9:9ab3:b558:ef1]) by AS8PR01MB7672.eurprd01.prod.exchangelabs.com
 ([fe80::c1b9:9ab3:b558:ef1%6]) with mapi id 15.21.0202.018; Tue, 14 Jul 2026
 09:49:39 +0000
MIME-Version: 1.0
Content-Type: text/plain
Date: Tue, 14 Jul 2026 09:49:39 +0000
Message-ID:
	<

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Answer 1

The email delivery failure with error 550 5.1.8 Access denied, bad outbound sender AS(42004) indicates that Microsoft's Exchange Online Protection (EOP) has blocked the message because the sending domain or IP address lacks proper authentication records or has a poor sender reputation. To resolve this, verify that your domain's SPF, DKIM, and DMARC records are correctly configured and published in DNS to authorize your sending infrastructure. Additionally, ensure that the sending IP address is not listed on any blocklists and, if using a third-party relay or application, confirm that it is properly authenticated through Exchange Online via a connector or SMTP AUTH. If the issue persists after correcting DNS records, submit a delisting request through the Microsoft Defender portal or contact Microsoft Support to review the sender reputation.