How to improve IP warmup with AT&T and resolve low open rates?
Michael Ko
Co-founder & CEO, Suped
Published 5 May 2025
Updated 18 Aug 2025
7 min read
Starting with a new IP address for email sending is a critical phase known as IP warming. It involves gradually increasing your sending volume to build a positive reputation with mailbox providers. When this process hits a snag, especially with specific internet service providers (ISPs) like AT&T, it can be incredibly frustrating. I have seen situations where initial open rates, even to engaged lists, drop dramatically, signalling a significant deliverability challenge.
You might be seeing healthy open and click rates with major providers like Google and Microsoft during your IP warmup, only to find AT&T domains (including att.net, sbcglobal.net, and bellsouth.net) perform poorly. This disparity suggests that the problem might not be a typical IP warming issue, but rather something more specific to AT&T's backend processing or your data.
Understanding AT&T's mail infrastructure
Many smaller broadband providers, including AT&T, do not manage their own extensive email filtering systems. Instead, they often route their email traffic through the infrastructure of larger mailbox providers. For AT&T, this often means that mail is passed to Yahoo's backend for inboxing. This distinction is crucial because it suggests that if your Yahoo and Ymail domains are performing well, your AT&T deliverability should theoretically follow a similar pattern.
The primary role of AT&T's own mail servers (or their MX infrastructure) is to accept incoming mail. After this initial acceptance, much of the heavy lifting for spam filtering and inbox placement is offloaded to Yahoo's robust systems. This means that if you're experiencing severe low open rates with AT&T but good performance with Yahoo, the issue might not be a direct blocking or temporary failure at the AT&T gate. Instead, it could be related to how Yahoo's filters perceive your mail, or more likely, issues with the recipient list itself.
AT&T does perform some basic filtering, primarily for malicious content, they generally do not have the same sophisticated engagement-based filters as Google, Microsoft, or Yahoo directly. This implies that if your mail is accepted by AT&T, it should largely follow the same inboxing fate as your Yahoo mail, provided your sending practices are consistent and reputable across both platforms.
Diagnosing low open rates beyond warming
When you encounter low open rates that aren't accompanied by high bounce rates or blocks, especially on domains like AT&T, it's crucial to look beyond the typical IP warming checklist. One common culprit is list quality. If you've migrated from an old ESP and are relying on a list export that includes 'last open' or 'last click' dates, there's a possibility that the data is not as clean as it seems. Some ESPs might include 'auto-opens' or bot activity in their engagement metrics, inflating perceived list quality. This could mean you're sending to many inactive or non-existent addresses.
The most telling sign is a high volume of 'recipient does not exist' bounces, even if your overall bounce rate is low. This indicates that a significant portion of your list for AT&T domains might be invalid. Even if your validation service struggled with these specific domains due to their infrastructure, the bounces confirm the issue. Sending to non-existent addresses severely damages your sender reputation and can lead to emails being sent straight to the spam folder, explaining the abysmal open rates.
The danger of 'recipient does not exist' bounces
A high rate of 'recipient does not exist' bounces, particularly during IP warmup, signals poor list hygiene. It tells ISPs that your sending practices are questionable, as legitimate senders typically maintain clean lists. This can lead to a quick blacklisting (or blocklisting) and severe deliverability issues, even if other ISPs are currently performing well.
Best practices for general IP warmup
Even if the AT&T issue is unique, adhering to general IP warming best practices is non-negotiable for long-term email deliverability. This process is all about building trust with mailbox providers by demonstrating consistent, positive sending behavior. A steady increase in volume, starting with your most engaged subscribers, is fundamental. Avoid large spikes in sending, as these are red flags for spam filters.
Your initial sends should always target your most active and engaged audience. These are the recipients most likely to open, click, and interact positively with your emails. High engagement metrics from these early sends signal to ISPs that your mail is wanted and valuable. As your reputation builds, you can gradually expand to broader segments of your list.
It's also essential to send high-quality, relevant content during warmup. This isn't the time for promotional blasts to your entire list. Focus on content that has historically generated good engagement. Regularly monitor your performance metrics, including open rates, click rates, and bounce rates, for each increase in sending volume. Watch out for any sudden drops in engagement or increases in spam folder placements, as these indicate a problem.
Effective warming strategies
Start small: Begin with a low volume of emails to your most active subscribers.
Gradual increase: Slowly increase sending volume over 4-8 weeks, depending on your total list size.
Monitor engagement: Pay close attention to open rates, clicks, and complaint rates.
Quality content: Send highly engaging and relevant content that your audience expects.
Common pitfalls to avoid
Sudden volume spikes: Sending too much too soon can trigger spam filters and IP blacklisting.
Poor list hygiene: Sending to invalid or unengaged addresses harms your reputation.
Ignoring feedback loops: Failing to process unsubscribes and complaints quickly.
Inconsistent content: Switching between different content types during warmup can confuse ISPs.
Essential email authentication and reputation management
Strong email authentication is a cornerstone of good deliverability, irrespective of the ISP. Ensure your Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM), and Domain-based Message Authentication, Reporting & Conformance (DMARC) records are correctly configured. These protocols prove to mailbox providers that you are a legitimate sender and that your emails haven't been tampered with. Misconfigurations here can lead to immediate rejection or spam folder placement, even if your IP is warmed.
Beyond technical setup, actively monitoring your sender reputation is paramount. Keep an eye on blocklists (or blacklists) and maintain a clean email list by regularly removing unengaged subscribers and invalid addresses. High spam complaint rates or being listed on a major blacklist can severely impact your deliverability across all ISPs, including AT&T.
Views from the trenches
Best practices
Focus on engagement: Send only to your most engaged subscribers during the initial warmup phases.
Consistency is key: Maintain a consistent sending schedule and content type.
Quality over quantity: Prioritize delivering valuable content that recipients want to open.
Monitor feedback: Pay attention to bounce rates, complaint rates, and spam folder placement.
Leverage authentication: Properly configure SPF, DKIM, and DMARC to build sender trust.
Common pitfalls
Sending to an unsegmented list during warmup, hitting dormant or invalid addresses.
Ignoring 'recipient does not exist' bounces, which indicate severe list quality issues.
Overlooking the specific routing behaviors of smaller ISPs like AT&T (through Yahoo's infrastructure).
Failing to implement or properly configure email authentication protocols.
Neglecting to monitor blocklists and sender reputation metrics during the warming phase.
Expert tips
Ensure precise list segmentation for warming, targeting only the most active users first.
Implement a slow and steady volume increase rather than aggressive jumps, especially for new IPs.
Regularly clean your email lists to remove inactive or invalid addresses, which can quickly damage reputation.
Leverage email authentication (SPF, DKIM, DMARC) for all sending domains to build trust with ISPs.
Monitor your deliverability metrics closely, looking for spikes in bounces or drops in engagement.
Expert view
Expert from Email Geeks says IP warming is primarily relevant for large mailbox providers. Smaller broadband providers like AT&T do not have the same engagement-based filters as Google or Yahoo. If you are not seeing blocks or temporary failures, then the low open rates might not be a deliverability or warming problem.
January 22, 2024 - Email Geeks
Expert view
Expert from Email Geeks says that AT&T's email infrastructure often uses Yahoo's backend for inboxing, meaning that if your Yahoo deliverability is good, AT&T's should be similar. The MX infrastructure for AT&T domains is distinct from Yahoo's inboxing infrastructure.
January 22, 2024 - Email Geeks
Moving forward with AT&T and beyond
Improving IP warmup with AT&T and resolving low open rates often requires a multi-faceted approach. While direct IP warming tactics are crucial, the specific issue with AT&T might stem from list hygiene problems ratherSocketsrather than a lack of IP reputation, especially if your Yahoo performance is robust. Addressing 'recipient does not exist' bounces is paramount, as these indicate a fundamental issue with your recipient data and can severely impact your sender reputation.
By combining rigorous list cleaning, sending highly engaged content, and ensuring all your email authentication protocols are perfectly aligned, you can build a strong sender reputation that transcends individual ISP quirks. Remember, the goal is not just to send emails, but to ensure they land in the inbox and are opened by engaged subscribers, setting the stage for long-term email marketing success.
AT&T does perform some basic filtering, primarily for malicious content, they generally do not have the same sophisticated engagement-based filters as Google, Microsoft, or Yahoo directly. This implies that if your mail is accepted by AT&T, it should largely follow the same inboxing fate as your Yahoo mail, provided your sending practices are consistent and reputable across both platforms.
