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How to Fix Error 550 5.7.1 in Microsoft 365 Email Delivery
Question details
Users are experiencing email delivery failures with Error 550 5.7.1 when sending messages from Microsoft 365 to AT&T-managed domains.
- Product
- Microsoft 365
- Device & OS
- not provided
- Scenario
- Sending outbound emails from Microsoft 365 to external email providers such as att.net, sbcglobal.net, and Prodigy.
- Observed behavior
- The receiving server rejects the connection with the message 'Connections not accepted from servers without a valid sender domain', typically due to reverse DNS or PTR-record requirements.
Ensure you have administrative access to your domain's DNS settings and keep the Non-Delivery Report (NDR) accessible, as it contains crucial troubleshooting details like timestamps and sending IP addresses.
Verify and Update SPF, DKIM, and DMARC Records
Ensure your domain's email authentication records are correctly configured to prove to strict receiving servers that your outbound emails are legitimate.
While PTR-record requirements are largely outdated, ensuring your modern authentication protocols (SPF, DKIM, and DMARC) are perfectly aligned can sometimes help mitigate aggressive spam filters on receiving networks.
Log in to the control panel of your domain registrar or DNS hosting provider and navigate to the DNS records management section.
Locate your TXT records and ensure your SPF record includes the required Microsoft 365 value: v=spf1 include:spf.protection.outlook.com -all.
Log in to the Microsoft 365 Defender portal, go to Email & Collaboration, select Policies & Rules, click on Threat policies, and verify that DKIM signatures are enabled for your domain.
Ensure a DMARC TXT record is published in your DNS settings (e.g., v=DMARC1; p=quarantine) and that it aligns properly with your SPF and DKIM configurations.
Escalate the Issue to Microsoft Support
Since Microsoft manages the outbound IP addresses for Microsoft 365, you must contact their support team to resolve IP-level blocks initiated by receiving domains.
Switch to WPS Office for a Seamless Document Experience
While resolving Microsoft 365 email server issues requires patience, managing your daily documents shouldn't. WPS Office provides a free, lightweight, and highly compatible alternative to Microsoft Office, letting you edit Word, Excel, and PowerPoint files without the subscription hassle.
- 1. Download the installer: Visit the official WPS Office website and download the free installer for your operating system.
- 2. Install the software: Run the downloaded file and follow the on-screen prompts to complete the quick installation process.
- 3. Open your files: Launch WPS Office and directly open your existing Microsoft Office documents to continue working without formatting loss.
Frequently Asked Questions
Why am I getting Error 550 5.7.1 specifically when sending to AT&T domains?
This error occurs because AT&T-managed domains (such as att.net, sbcglobal.net, and Prodigy) often enforce strict reverse-DNS or PTR-record checks. If the specific Microsoft 365 outbound IP address handling your message doesn't meet these requirements, AT&T's servers reject the connection.
Can I fix the PTR-record issue myself in the Microsoft 365 admin center?
No. Because Microsoft manages the shared pool of outbound IP addresses for all Microsoft 365 tenants, you cannot manually configure PTR records for them. You must rely on Microsoft Support to address the IP block with the receiving provider.
What information should I provide to Microsoft Support for this error?
When contacting support, provide the complete Non-Delivery Report (NDR) attached to the bounce message. Ensure it includes the exact timestamp of the failed delivery, the recipient's email domain, and the specific sending IP address that was rejected.
Are PTR-record requirements still considered a standard practice for email delivery?
Requiring a PTR record for inbound email delivery is largely considered an outdated practice and was deprecated by many major providers around 2014. However, some legacy systems and specific providers like AT&T still enforce it, leading to temporary delivery interruptions.
