Summary
Email throttling from major inbox providers like Gmail and Microsoft can be a perplexing challenge, especially when your engagement rates are high and your sender reputation appears solid. This phenomenon occurs when ISPs (Internet Service Providers) temporarily limit the number of emails your server or IP address can send within a specific timeframe, leading to delays and extended queue times. While it often signals a perceived issue with sending patterns or content, it can sometimes appear without obvious changes in your sending behavior. Understanding the underlying reasons and effective mitigation strategies is crucial for maintaining optimal email deliverability and ensuring your messages reach the inbox promptly.
Key findings
- Sudden onset: Throttling can occur unexpectedly, even if sending volume and cadence remain consistent over extended periods.
- Microsoft's irregular traffic flag: Microsoft (Outlook, Hotmail) may flag "irregular mail traffic" patterns, suspecting compromise or sudden changes, even when none have occurred.
- Dedicated IP vs. throttling: Having a dedicated IP address does not automatically prevent throttling, as ISPs still apply limits based on sending behavior and reputation.
- Beyond engagement and reputation: Despite high open rates, strong domain and IP reputation, and low abuse rates, throttling can still happen, indicating other factors are at play.
- Postmaster interaction: Direct communication with ISP postmaster teams, providing comprehensive data, is often necessary.
Key considerations
- Monitor sender performance: Regularly check your metrics in tools like Microsoft's SNDS and Google Postmaster Tools to catch any anomalies.
- Persistence with postmaster support: Generic responses from support are common, requiring persistent follow-up with data and escalation requests.
- Data-backed appeals: Provide detailed reports on consistent volume, high engagement, and low abuse rates to support your case.
- Review sending patterns: Even subtle changes, like batching larger sends, can sometimes trigger flags. For more insights, refer to this guide on email throttling.
- Proactive management: Implement continuous list hygiene and engagement filtering to minimize potential issues.
What email marketers say
Email marketers often face unexpected challenges with deliverability, even when adhering to best practices. Instances of throttling by major ISPs like Gmail and Microsoft, despite strong engagement and reputation, are a recurring concern. These experiences highlight the unpredictable nature of ISP filtering algorithms and the importance of proactive communication with postmaster teams.
Key opinions
- Microsoft issues: Many marketers report recent and sudden throttling issues specifically with Microsoft (Outlook/Hotmail) domains.
- SNDS importance: Checking Microsoft's Smart Network Data Services (SNDS) for IP status is a common first step, though green or yellow statuses don't always prevent throttling.
- Generic postmaster responses: It's common to receive initial generic responses from postmaster support indicating "mitigation not possible".
- False flag concerns: Some believe these issues might be false flags or anomalies from the inbox providers, especially when sender behavior is consistent.
- Escalation as a strategy: Escalating the support case with detailed data is often the only way to get a deeper review.
Key considerations
- Document everything: Keep thorough records of sending volume, cadence, engagement metrics, and abuse rates to present to ISP support. This data is critical when addressing Microsoft Outlook deliverability issues.
- Push back on generic replies: Don't accept "mitigation not possible" without providing your own data and requesting further investigation.
- Community insights: Share experiences with other marketers to identify widespread anomalies, as discussed in various email marketing forums.
- Engage and filter: Continuously filter your sending list based on engagement to minimize potential negative signals, which can improve deliverability to Microsoft domains.
Marketer view
Email Geeks Marketer indicates a recent increase in rate-limiting and throttling at Gmail and Microsoft, even for clients with high open rates and strong domain/IP reputation. They noted queue times of around 15 minutes, which is unusual for a dedicated IP. This client's volume and cadence remained unchanged, aside from splitting a large send into two batches. They questioned if other marketers were experiencing similar anomalies.
Marketer view
Email Geeks Marketer reported experiencing recent issues specifically with Microsoft, but not Gmail. They highlighted the importance of checking SNDS (Smart Network Data Services) for the IP status. This suggests that while overall reputation might be good, Microsoft's internal systems could be flagging specific traffic patterns. They noted that their SNDS showed mostly green statuses with only a couple of yellow days, indicating that visual indicators aren't always definitive.
What the experts say
Email deliverability experts consistently point out that ISP algorithms are complex and dynamic, often reacting to subtle shifts in sending patterns, even when a sender maintains a strong reputation and high engagement. Throttling is a protective measure by ISPs, designed to prevent potential abuse or to manage traffic flow, and it's not always a direct indicator of bad sending practices. Expert advice focuses on forensic analysis of logs, detailed communication with ISPs, and a deep understanding of evolving sender requirements.
Key opinions
- Algorithmic complexity: ISP algorithms consider numerous factors beyond simple engagement metrics, including historical patterns, recipient behavior, and even infrastructure changes.
- Beyond surface metrics: While good open rates are positive, ISPs also scrutinize spam complaint rates, unknown user rates, and bounce rates, which can impact deliverability even with high engagement. This is critical for Gmail deliverability.
- False positives exist: Sometimes, throttling is a false positive or a temporary anomaly on the ISP's side, requiring direct intervention and data presentation.
- Reputation is dynamic: Sender reputation is not static; it constantly fluctuates based on recent sending behavior, list quality, and feedback loops.
- Escalation pathways: Successful resolution often involves navigating through standard support to higher-tier escalation teams with robust data.
Key considerations
- Deep dive into logs: Analyze sending logs for specific error codes or patterns that precede throttling events. This can help improve Gmail deliverability rates.
- Segment and warm-up: Even with existing reputation, significant changes in volume or list segmentation might require a renewed warming-up period, as detailed in deliverability best practices.
- Technical alignment: Ensure all authentication records (SPF, DKIM, DMARC) are correctly configured and aligned, as misconfigurations can indirectly contribute to throttling.
- Feedback loop monitoring: Actively monitor feedback loops for spam complaints and remove non-engagers promptly.
Expert view
Email Geeks Expert suggests that throttling, even for high-reputation senders, can be a symptom of a hidden issue that algorithms are designed to detect. They recommend looking beyond obvious metrics like open rates and into the granular details of SMTP logs for clues.
Expert view
Spam Resource contributor advises that maintaining a consistent sending volume and cadence is paramount, but even slight deviations can be flagged by ISPs. They emphasize that "irregular mail traffic" warnings from Microsoft are often an indicator of perceived inconsistency, even if the sender believes their volume is stable.
What the documentation says
Official documentation from major email providers like Google and Microsoft often outlines general principles for good sender reputation and best practices, though specific throttling triggers are rarely detailed explicitly. These documents emphasize consistent sending patterns, low complaint rates, proper authentication, and positive user engagement as foundational for good deliverability. When throttling occurs despite these, it suggests that algorithms are detecting a nuanced pattern deviation or a perceived risk.
Key findings
- Traffic pattern monitoring: ISPs continuously monitor sending patterns for stability and predictability. Sudden, unexplained spikes in volume or changes in recipient types can trigger scrutiny.
- Feedback loop importance: Official documentation stresses the importance of signing up for and promptly acting on feedback loops (FBLs) to minimize spam complaints.
- Authentication standards: Strict adherence to authentication protocols such as SPF, DKIM, and DMARC is fundamental for establishing sender trust, as highlighted in various ISP guidelines.
- User interaction signals: ISPs use complex metrics beyond open rates, including deletes without opening, 'mark as not spam' actions, and moves to folders, to assess user engagement.
Key considerations
- Adhere to best practices: Continually review and adapt your sending practices to align with the latest Microsoft Outlook sender requirements and Gmail's guidelines for bulk senders.
- Leverage postmaster tools: Utilize Microsoft's SNDS and Google Postmaster Tools for insights into your domain and IP reputation, spam complaint rates, and delivery errors.
- Understand warning messages: Pay close attention to generic warning messages from ISPs, as they often contain subtle clues, like "irregular mail traffic", even if they don't explicitly state the cause of throttling.
Technical article
Google Postmaster Tools Documentation states that reputation is based on a sender's sending history, and any significant, uncharacteristic change can lead to adjustments in filtering, including temporary rate limits. This includes sudden increases in volume, new IP usage, or changes in content that deviate from typical patterns.
Technical article
Microsoft's Sender Support documentation explains that email traffic that deviates from established patterns can be perceived as suspicious, even if it's benign. Their systems are designed to detect potential compromises, and atypical volume or recipient behavior can trigger throttling as a precautionary measure.
Related resources
1 resources
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