Why are AT&T email domains experiencing a spike in block bounces?
Michael Ko
Co-founder & CEO, Suped
Published 2 Jul 2025
Updated 13 Oct 2025
8 min read
Recently, many of us sending emails, especially those targeting AT&T, SBCGlobal.net, and Bellsouth.net domains, have observed a concerning spike in block bounces. This isn't just affecting a few senders; it appears to be a broader issue, impacting even those with historically strong sending reputations. When emails that usually deliver successfully suddenly get rejected, it signals a shift in recipient server policies or a widespread issue affecting many senders.
The frustration is palpable. One day, your campaigns are running smoothly, and the next, you're hit with high bounce rates, indicating your messages are being blocked before they even reach the inbox. What makes this particularly challenging is that these bounce messages can sometimes take days to arrive, unlike typical bounces that provide immediate feedback. This delay can obscure the problem, making it harder to diagnose and address quickly.
We understand the urgency of this situation. Email deliverability is crucial for business operations, marketing efforts, and critical communications. In this article, we'll explore the likely reasons behind this spike in block bounces from AT&T domains and provide actionable steps you can take to troubleshoot and hopefully restore your email delivery.
Understanding AT&T's email filtering landscape
AT&T, like other major internet service providers (ISPs), employs sophisticated spam and security filters to protect its users. These systems are constantly evolving, and a sudden change or update can significantly impact email deliverability for many senders. Their filtering often relies on a combination of internal proprietary blocklists (or blacklists) and external real-time blocklists (RBLs) or DNS-based blocklists (DNSBLs).
One common indicator of this type of blocking is a bounce message containing phrases such as 'DNSBL:RBL 521' or similar references to a blocklist entry. This suggests that your sending IP address or domain has been identified as a source of potentially unwanted email, even if your sending practices are otherwise legitimate. It’s a clear signal that your email server or domain is on a blocklist (or blacklist) somewhere in their system.
The challenge with AT&T, SBCGlobal, and Bellsouth domains is that their internal systems can be particularly stringent. If they detect a perceived threat or a significant change in sending patterns, they may implement temporary or even more permanent blocks that are not easily resolved by simply reducing volume. We’ve seen instances where even senders with sterling reputations, meticulously maintaining their lists and adhering to best practices, face these hurdles.
Example of a block bounce message from AT&Tplaintext
5.3.0 (unknown mail system-related status) flpd569 DNSBL:RBL 521<your_ip_or_domain>_is_blocked.For assistance forward this email to abuse_rbl@abuse-att.net.
Investigating the root causes of the spike
Several factors could contribute to a sudden spike in block bounces from AT&T domains. While a global shift in their filtering policy is a possibility, it's also crucial to examine your own sending practices and infrastructure. Even minor changes can sometimes trigger alarms with sensitive email providers.
Common issues
Sender reputation decline: A recent increase in spam complaints, sending to old or unengaged lists, or hitting spam traps can rapidly degrade your reputation. High bounces are viewed negatively by mailbox providers.
IP or domain blocklisting: Your sending IP or domain might have appeared on a private or public email blocklist (also known as a blacklist), leading to immediate rejection by AT&T's mail servers. You can use a blocklist checker to verify this.
Content flags: Certain keywords, links, or formatting within your email content could be triggering AT&T's spam filters.
Authentication misconfiguration: Issues with your SPF, DKIM, or DMARC records can lead to emails being rejected, especially by strict providers. For instance, reverse DNS failure bounces from AT&T are often authentication-related.
Diagnostic steps
Analyze bounce messages: Look for specific error codes or explanations. The 'DNSBL:RBL' message is a key indicator that your IP is on a blocklist.
Check public blocklists: Use a blocklist monitoring tool to see if your sending IPs or domains are listed. While AT&T uses its own internal lists, a listing on major public RBLs often precedes or accompanies an internal block.
Review authentication records: Ensure your SPF, DKIM, and DMARC records are correctly configured and aligned. A simple error here can cause widespread delivery issues.
Examine sending patterns: Consider if there's been any significant increase in volume or change in content that coincided with the spike in bounces.
The fact that many US-based senders are reporting similar issues, regardless of their email service provider (like Salesforce Marketing Cloud), points towards a potential systemic change at AT&T. However, this doesn't absolve senders from diligent self-auditing. We must ensure our own house is in order before assuming it's solely on the recipient side. If you're experiencing a sudden spike in email bounce rates it always warrants a thorough investigation.
It's worth noting that some providers may employ data limiting, sending only a certain amount of emails from a single sender to specific clients to limit data usage. This isn't necessarily a blocklist but can mimic block bounces if you exceed these hidden thresholds.
Steps to resolve and improve deliverability
Once you've diagnosed the likely cause of the block bounces, it's time to take action. Recovering from an IP or domain blocklist (blacklist) and improving your deliverability to AT&T requires a systematic approach. Here's what you can do:
Contact AT&T postmaster: If your bounce messages indicate a specific AT&T blocklist or DNSBL, forward the full bounce message to abuse_rbl@abuse-att.net. This is often the most direct route for requesting removal, as detailed in guides like removing your IP from AT&T's blocklist.
Address underlying issues: Before contacting them, ensure you've fixed any issues that might have led to the block. This includes cleaning your email lists, addressing spam complaints, and verifying your email authentication. See our guide on how to fix emails going to spam for more details.
Warm up new IPs (if applicable): If you're on a new IP or a shared IP pool, proper IP warming is critical to build trust with ISPs like AT&T.
Review email content: Remove any potentially spammy language, excessive links, or attachments that could trigger filters. Always consider a professional, non-promotional 'friendly from' name.
The impact of DMARC on deliverability
Ensuring your DMARC, SPF, and DKIM records are correctly configured is paramount. Many ISPs, including AT&T, increasingly rely on these authentication protocols to verify sender legitimacy. A misconfigured DMARC record or a lack of proper alignment can lead to emails being sent directly to spam or blocked entirely. Use a DMARC monitoring solution to gain visibility into your email authentication status and identify potential issues.
While throttling volume might seem like a quick fix, some senders have found it doesn't always resolve the issue with AT&T's block (or blacklist). The key is often to identify and rectify the underlying cause of the block, rather than just reducing the sending rate. Persistent blocks suggest a deeper reputation issue or a specific policy trigger that needs direct attention.
Monitoring and long-term deliverability strategies
Even after resolving a block, continuous monitoring is essential to prevent future issues. The email landscape is constantly changing, and what works today might not work tomorrow. This proactive approach ensures your email program remains healthy and your messages consistently reach the inbox.
Implement robust list hygiene: Regularly clean your email lists to remove inactive or invalid addresses, reducing your bounce rate and avoiding spam traps. This also helps improve your overall email domain reputation.
Utilize DMARC reporting: DMARC reports provide invaluable insights into how your emails are being authenticated and handled by recipient servers. Use this data to identify and fix authentication failures.
Monitor blocklists (blacklists): Keep a close eye on major public RBLs. While AT&T has its own internal blocklists, a listing on a prominent public list often impacts delivery across many providers, including AT&T.
Maintain consistent sending: Avoid sudden spikes or drops in sending volume, which can appear suspicious to ISPs. Consistent volume builds trust and a reliable sender reputation.
Proactive email deliverability testing
Regularly using an email deliverability tester can help you identify potential issues before they impact your campaigns. These tools can check your email content, authentication, and IP reputation against various criteria used by major mailbox providers. It's a key part of boosting email deliverability rates.
Maintaining a strong sender reputation is an ongoing effort. It involves not just technical configuration but also responsible sending practices, focusing on sending desired content to engaged recipients. This dedication pays off in consistent inbox placement and fewer unexpected blocks from providers like AT&T.
Views from the trenches
Best practices
Regularly monitor bounce messages for specific error codes to quickly identify blocklisting issues.
Always maintain impeccable list hygiene to reduce bounces and avoid spam traps.
Ensure SPF, DKIM, and DMARC are correctly configured for robust email authentication.
Proactively test email deliverability to various providers before major campaign sends.
Common pitfalls
Assuming block bounces are always temporary and will resolve themselves without intervention.
Failing to analyze bounce messages, missing critical information about the block source.
Not contacting AT&T's postmaster (abuse_rbl@abuse-att.net) directly for block removals.
Continuing to send high volumes of mail without addressing the underlying deliverability problem.
Expert tips
Even senders with a stellar reputation can experience sudden blocks from AT&T, indicating potential changes in their filtering.
Sometimes, throttling email volume won't help resolve AT&T blocks; direct removal requests are often necessary.
Pay close attention to 'DNSBL:RBL 521' in bounce messages, as it indicates a blocklist entry at AT&T.
The issue of increased AT&T blocks appears to be a global concern affecting many US-based senders.
Marketer view
Marketer from Email Geeks says they noticed a significant increase in block bounces for AT&T domains, especially when using Salesforce Marketing Cloud.
2019-07-11 - Email Geeks
Marketer view
Marketer from Email Geeks says their client, a high-reputation sender, is experiencing a large number of blocks and bounces, despite attempting to throttle volume at the MTA level. They suggested checking for mentions of RBLs in bounce messages.
2019-07-11 - Email Geeks
Restoring your email stream
The recent spike in block bounces from AT&T, SBCGlobal.net, and Bellsouth.net domains underscores the dynamic nature of email deliverability. Even with a strong sender reputation, unforeseen changes in ISP filtering policies can disrupt your email flow. The key is to remain vigilant, understand the bounce messages, and act swiftly to identify and address the root causes.
While direct communication with AT&T's postmaster team (abuse_rbl@abuse-att.net) is often necessary for removal from their internal blocklists, this must be coupled with rigorous self-auditing of your email program. Ensure your authentication records are sound, your lists are clean, and your sending practices align with industry best practices for optimal inbox placement. For more insights on this topic, refer to our comprehensive guide on what happens when your IP gets blocklisted.
By proactively monitoring your deliverability metrics, promptly addressing bounce feedback, and maintaining a high-quality sending program, you can navigate these challenges and ensure your important messages reach their intended recipients, even amidst evolving ISP filters.
