Why are email clients being blocked by AT&T and how was it resolved?
Michael Ko
Co-founder & CEO, Suped
Published 13 May 2025
Updated 17 Aug 2025
5 min read
Recently, many email clients started experiencing significant email blocking issues when attempting to send messages to AT&T associated domains, including att.net, sbcglobal.net, and bellsouth.net. This wasn't just isolated to a few senders; it appeared to be a widespread phenomenon affecting a diverse range of platforms and services.
The immediate impact was concerning: emails were bouncing with various error messages, indicating that AT&T's servers were refusing connections or rejecting messages outright. For businesses and individuals relying on email for critical communications, this disruption led to significant frustration and potential operational halts.
Initially, many suspected common deliverability culprits like IP blacklists (or blocklists) or sender reputation issues. However, the sheer scale and randomness of the blocks, affecting even well-configured sending domains and IPs, suggested a deeper, more systemic problem. It quickly became clear that this was not a typical email deliverability challenge.
The sudden surge in blocks
The problem escalated over a weekend, with reports flooding in from different email service providers and companies. Senders observed that their clients, operating on entirely separate platforms, began encountering issues concurrently. This synchronized failure across varied sending infrastructures reinforced the idea that the problem lay with the recipient ISP, AT&T.
What made this situation particularly puzzling was the inconsistent nature of the impact. While many IPs and domains were completely blocked, some clients or specific IP addresses within a sender's infrastructure remained unaffected. This made initial troubleshooting efforts difficult, as typical deliverability fixes weren't yielding results. This behavior diverged significantly from standard email blacklist mechanisms.
Reports from various sources, including communities and forums, confirmed that these widespread issues started around Friday or Saturday, indicating a sudden, system-wide change rather than gradual reputation degradation. This pointed towards an infrastructure-level event rather than a localized sender problem.
Investigating the root cause
When confronted with such unusual blocking patterns, the most crucial step is to engage directly with the affected ISP. In this case, reaching out to the AT&T postmaster team was paramount. Many in the email community promptly sent detailed emails to abuse_rbl@abuse-att.net, providing their affected IP addresses and domain information.
The critical question was whether the failure was occurring on AT&T's receiving servers or further downstream after initial acceptance. This distinction is vital for accurate troubleshooting. If emails were being rejected at the connection phase, it implied a network or primary filtering issue. If they were accepted and then disappeared or bounced later, it could point to internal routing or content-based filtering.
Fortunately, key contacts within AT&T (and its associated partner, Yahoo, which handles AT&T email infrastructure), were able to investigate the issue internally. This direct line of communication with AT&T's postmaster proved invaluable in getting to the bottom of the widespread blocking.
AT&T's network outage and its impact
The official explanation that emerged was that AT&T had experienced a major network outage over the weekend. This outage wasn't specific to email services but affected broader network infrastructure. As a security measure or an unintended consequence of the network disruption, email traffic was inadvertently blocked on a large scale.
This situation differed significantly from typical email blocking scenarios, where sender reputation, content, or authentication failures (like DMARC issues) lead to blocks. In this case, legitimate emails from reputable senders were caught in a system-wide block due to an infrastructure problem at AT&T's end. One report indicated that AT&T email servers were blocking connections from Microsoft 365 due to a high volume of spam originating from Microsoft’s service.
This highlights a crucial distinction: not all email blocks are due to poor sender practices. Sometimes, external factors like ISP network instability or even overly aggressive spam filtering can cause legitimate email to be rejected. Understanding why AT&T might block emails is key to effective troubleshooting.
The resolution and long-term prevention
Thankfully, the resolution came relatively quickly. Once AT&T resolved their underlying network outage, the email blocking issues subsided. Many senders reported that traffic to AT&T domains returned to normal without specific intervention from their end to remove an IP from a blacklist. This confirmed that the problem was indeed external and resolved by the ISP.
This event underscores the importance of staying informed and having robust monitoring in place. While you can't prevent an ISP from having an outage, rapid detection and understanding of the root cause allow you to communicate effectively with your customers and avoid unnecessary, time-consuming troubleshooting on your side. Having a good communication channel with postmasters is invaluable.
Typical email blocking
Cause: Often due to sender reputation issues, spam complaints, invalid recipients, or misconfigured authentication (SPF, DKIM, DMARC).
Impact: Specific IP addresses or domains may be added to public or private blocklists. Bounces often provide specific error codes related to spam or policy violations.
Resolution: Requires sender-side actions like improving sending practices, requesting delisting, or addressing technical misconfigurations.
AT&T outage scenario
Cause: A major network infrastructure outage at the ISP's end, affecting various services, including email delivery.
Impact: Widespread, often indiscriminate blocking of legitimate email due to system instability or emergency filtering protocols. Bounces might be generic connection failures.
Resolution: Primarily handled by the ISP. Senders should monitor the situation and can contact the postmaster to provide data, but direct IP delisting isn't typically required.
Maintaining email deliverability resilience
While we cannot control ISP outages, we can implement robust deliverability practices to ensure resilience. A strong email deliverability strategy involves continuous monitoring and adherence to best practices, which minimize your risk during such events.
Views from the trenches
Best practices
Maintain diverse sending IPs, if possible, to avoid single points of failure during an ISP outage.
Regularly monitor your email bounce rates and delivery logs for unusual spikes or patterns.
Establish direct communication channels with major ISPs and their postmaster teams.
Segment your email lists to reduce overall impact if one segment encounters an issue.
Common pitfalls
Assuming all blocks are sender-reputation related without investigating broader network issues.
Failing to communicate with affected recipients about potential delivery delays or issues.
Not having a contingency plan for email delivery disruptions to critical domains.
Overlooking ISP status pages and industry forums during widespread deliverability problems.
Expert tips
Subscribe to major ISP status updates and industry newsletters for proactive outage alerts.
Develop internal protocols for crisis communication when email delivery is compromised.
Educate your team on differentiating between sender-caused blocks and ISP-side issues.
Leverage DMARC reports to gain deeper insights into why emails are failing authentication at ISPs.
Marketer view
Marketer from Email Geeks says they started seeing clients on totally different platforms affected by AT&T blocking, beginning last Friday.
2023-08-01 - Email Geeks
Expert view
Expert from Email Geeks says AT&T had a major outage over the weekend and suggests emailing them IPs for checking.
AT&T.
Yahoo, which handles AT&T email infrastructure), were able to investigate the issue internally. This direct line of communication with AT&T's postmaster proved invaluable in getting to the bottom of the widespread blocking.
AT&T had experienced a major network outage over the weekend. This outage wasn't specific to email services but affected broader network infrastructure. As a security measure or an unintended consequence of the network disruption, email traffic was inadvertently blocked on a large scale.
