Send & Receive Issues

Fix Outlook 550 5.7.1 Email Rejection for Invalid AT&T Sender Domain

Maira Mehtab Sep 22, 2026 869 views

Question details

Users are experiencing bounce-back messages when attempting to send emails to AT&T-affiliated domains, receiving a 550 5.7.1 rejection error.

Product: Email Server / Microsoft Outlook
Device & OS: not provided
Scenario: Sending outbound emails to AT&T-managed email addresses such as att.net, sbcglobal.net, or prodigy.net.
Observed behavior: Emails are rejected and returned with a 550 5.7.1 Non-Delivery Report (NDR) stating that the sending server lacks a valid sender domain or reverse-DNS (PTR) information.

Before you start

Ensure you have administrative access to your domain's DNS settings or contact your IT department, and verify that your outgoing mail server IP address is not listed on any public blacklists.

Solution 1Recommended

Verify and Update Email Authentication and DNS Records

Ensure your domain has correctly configured reverse-DNS (PTR) and email authentication records (SPF, DKIM) to comply with AT&T's strict receiving policies.

AT&T mail servers enforce strict sender verification to block spam. If your email server (including Microsoft sending servers) does not have a valid reverse-DNS (PTR) record, AT&T will automatically reject the incoming messages.

PTR records map an IP address back to a domain name, proving that the server is authorized to send emails on behalf of that domain.

Check your domain's DNS records

Log into your domain registrar or DNS hosting provider's administrative dashboard to review your current DNS configuration.

Verify SPF and DKIM entries

Ensure your Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM) records are accurately published and include the IP addresses of your outgoing mail servers.

Configure Reverse-DNS (PTR)

Contact your Internet Service Provider (ISP) or server hosting provider to request a PTR record setup for your outbound email IP address. Only the entity that owns the IP address can create a PTR record.

DNS Propagation Delay: After updating your SPF, DKIM, or PTR records, it can take anywhere from 24 to 48 hours for the changes to propagate globally across the internet.

Solution 2

Escalate the Issue to AT&T or Microsoft Support

If your DNS settings are correct but emails from Microsoft servers are still being blocked by AT&T, escalate the issue directly to the administrators.

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Frequently Asked Questions

What does the 550 5.7.1 email bounce error mean?

The 550 5.7.1 error indicates that the recipient's email server (in this case, AT&T) rejected your message due to security or policy restrictions. Common causes include a missing reverse-DNS (PTR) record, an invalid sender domain, or your sending IP being placed on a blacklist.

Why is AT&T blocking emails sent from Microsoft servers?

AT&T sometimes enforces strict reverse-DNS/PTR checks on all incoming mail. If a specific Microsoft server IP lacks a proper PTR record, or if AT&T's automated filters mistakenly flag a Microsoft subnet as suspicious, legitimate emails can be blocked.

Can I set up a PTR record by myself?

Typically, no. A PTR record must be created by the organization that owns the IP address, which is usually your Internet Service Provider (ISP) or web hosting company. You will need to contact them to configure it for your outbound mail server.

What domains are affected by this AT&T rejection?

This strict filtering policy generally affects all AT&T-affiliated email domains, including att.net, sbcglobal.net, prodigy.net, bellsouth.net, and currently used AT&T Yahoo Mail addresses.

Outlook & Email Problems