Why has Microsoft email engagement dropped and how does throttling affect email deliverability?
Michael Ko
Co-founder & CEO, Suped
Published 29 Apr 2025
Updated 15 Aug 2025
8 min read
Email deliverability is a dynamic landscape, and few challenges are as perplexing as a sudden drop in engagement, especially with major providers like Microsoft. It's a common experience for email marketers and senders: your campaigns are performing well, and then, seemingly out of nowhere, open and click rates for Outlook and Hotmail addresses plummet. This can be particularly frustrating because initial investigations often yield no obvious culprits, leaving you searching for hidden factors.
One significant, yet often misunderstood, factor in this scenario is email throttling. While it may not immediately appear as a hard bounce or a spam folder placement, throttling can severely impact how many emails actually reach the inbox in a timely manner, and thus, your engagement metrics. It's a subtle signal from the receiving server, indicating that your sending volume or patterns are under scrutiny. Understanding why Microsoft might throttle your emails and how it affects your deliverability is crucial for maintaining a healthy email program.
Why Microsoft engagement drops
Microsoft, encompassing Outlook, Hotmail, and other services, employs sophisticated filtering systems designed to protect its users from unwanted mail. Their algorithms continuously evaluate incoming email streams based on various signals, including sender reputation, content quality, and user engagement. If any of these signals falter, even slightly, it can lead to immediate adjustments in how your emails are handled. Unlike some other mailbox providers, Microsoft's systems can be particularly sensitive to sudden changes or perceived inconsistencies in sending behavior, often leading to deliverability challenges, as some have noted. More on this here: Microsoft troubleshoot engagement drops.
One particular challenge I've observed is that while a decline in engagement might affect all major providers, Microsoft sometimes shows a disproportionately worse performance. For instance, an email campaign might see decent engagement with Gmail or Yahoo, but the same email could perform three to five times worse with Microsoft domains. This disparity suggests that there are specific nuances in Microsoft’s filtering or user engagement patterns that differ from other major players. It's not always about blacklisting or direct spam folder placement; sometimes, it's a more subtle form of suppression.
Delving into your email analytics, such as those provided by an email service provider or analytics tools, becomes critical here. It allows you to pinpoint exactly when engagement started to drop and which domains were most affected. If you notice a significant decline specifically in Microsoft engagement without a corresponding increase in bounce rates or clear spam complaints, it's a strong indicator that you might be dealing with a less overt issue, such as throttling or subtle filtering that affects inbox placement.
Causes for dropped engagement
Sender reputation: Poor reputation due to high bounce rates, spam complaints, or sending to unengaged users.
List hygiene: Sending to invalid or unengaged addresses, leading to spam traps or bounces.
How throttling affects deliverability
Email throttling is a control mechanism where an internet service provider (ISP) or mailbox provider temporarily limits the number of emails you can send to their users within a specific timeframe. This isn't necessarily a permanent block or blacklist (or blocklist) action, but rather a slowdown. ISPs use throttling to prevent server overload, manage spam, and evaluate sender behavior. If a sender suddenly sends a large volume of emails, or if their sending patterns deviate from established norms, throttling can be automatically triggered.
The critical distinction with throttling is that emails aren't immediately rejected. Instead, they are delayed, often for hours or even days. While they might eventually be delivered, this delay significantly impacts email engagement. If your marketing or transactional emails arrive too late, they lose their relevance, leading to lower open and click rates. This is particularly true for time-sensitive communications. An external resource provides a good overview of how email throttling works.
I've seen instances where email service providers reported throttled emails as soft bounces, which eventually got delivered. However, even if delivery ultimately occurs, the impact on engagement is undeniable. A throttled email reaching an inbox hours later might be seen after the user has already moved on, or it might be buried under newer, more timely messages. This can contribute to a perceived drop in engagement, even if your underlying deliverability metrics like bounce rates remain stable.
Immediate impact
Emails are delayed, not immediately delivered. This can lead to missed opportunities for time-sensitive campaigns and a degraded user experience. Throttling is a warning shot from Microsoft.
Recipient engagement (opens, clicks) drops due to delayed delivery. Users may have already checked their inbox or received the information elsewhere.
Long-term impact
Consistent throttling signals a poor sending reputation to ISPs. This can escalate to your emails being directed to the spam folder or even outright blocked in the future. Learn why Microsoft deliverability can be poor.
Recovery from a throttled state can be slow and requires consistent positive sending behavior over time. It impacts overall inbox placement across all campaigns.
Core factors influencing engagement and throttling
A primary driver of both dropped engagement and throttling is your sender reputation. This reputation is a score ISPs assign to your sending domain and IP address, based on your sending history. Factors like spam complaint rates, bounce rates, the presence of spam traps on your list, and consistent engagement all play a role. A sudden increase in any negative signal, or even a decrease in positive signals, can quickly lead to reputation degradation and subsequent throttling or even placement on a blacklist (or blocklist).
Beyond reputation, the relevance and quality of your email content are paramount. If your emails are not resonating with your audience, leading to low open and click rates, Microsoft’s systems might interpret this as a sign of disengagement or even unwanted mail. This can trigger filtering mechanisms that either divert your emails to the junk folder or apply throttling. It’s a feedback loop: low engagement leads to poorer placement, which in turn leads to even lower engagement, making recovery difficult.
Email authentication protocols such as SPF, DKIM, and DMARC are foundational for establishing trust with mailbox providers. Misconfigurations or failures in these protocols can significantly harm your deliverability. For instance, a hidden SPF DNS timeout or issues with DKIM temporary error rates specifically with Microsoft can lead to emails failing authentication checks, thus impacting both your sender reputation and increasing the likelihood of throttling or blocking. Regularly reviewing your Microsoft SPF DNS settings is essential.
Strategies to combat low engagement and throttling
To counteract dropped engagement and throttling, a multi-faceted approach is necessary. First and foremost, maintain impeccable list hygiene. Regularly clean your email lists to remove inactive or invalid addresses, and implement double opt-in to ensure subscribers genuinely want to receive your emails. This proactive measure reduces bounces and helps avoid spam traps, which are notorious for damaging sender reputation and triggering blocklists.
Monitoring your sender reputation is also non-negotiable. Use tools to track your IP and domain health, watch for any blocklist appearances (or blacklist appearances), and pay close attention to feedback loops provided by Microsoft and other major ISPs. These insights offer early warnings of potential issues, allowing you to address them before they escalate. For those sending to Microsoft domains, actively monitoring services like SNDS (Smart Network Data Services) can provide valuable feedback on your sending reputation and potential issues. This can help improve Hotmail and Outlook open rates.
Finally, focus on sending engaging content that provides value to your recipients. Personalize your emails, segment your audience, and encourage interaction to boost positive engagement signals. If you're introducing new IPs or significantly increasing sending volume, implement a gradual warm-up strategy to build trust with ISPs slowly. This involves sending small volumes initially and steadily increasing them over time. This approach helps prevent sudden throttling and establishes a consistent, positive sending history. For more on this, you can look at reasons for deliverability drops.
Maintain a clean email list by regularly removing inactive or invalid addresses to reduce bounce rates and avoid spam traps.
Implement double opt-in for all new subscribers to ensure explicit consent and higher engagement.
Monitor your sender reputation using Microsoft's Smart Network Data Services (SNDS) and other tools for early detection of issues.
Common pitfalls
Ignoring soft bounces, especially those related to throttling, as they can indicate impending delivery issues.
Failing to regularly review and update SPF, DKIM, and DMARC records, leading to authentication failures.
Sending to unengaged segments of your list, which signals low interest to ISPs and negatively impacts reputation.
Expert tips
If throttling occurs, pause sending to the affected domain, then resume with a lower volume and gradually increase.
Analyze engagement data by domain to identify specific providers causing issues, rather than looking only at overall metrics.
Engage directly with Microsoft's postmaster team if persistent deliverability problems arise after implementing best practices.
Marketer view
Marketer from Email Geeks says they investigated a sudden drop in Microsoft email engagement and found it was partly due to changes in user behavior during the pandemic, where webmail and mobile engagement declined.
2020-05-21 - Email Geeks
Marketer view
Marketer from Email Geeks says that despite no clear campaign or domain issues, the shift in user experience on Microsoft platforms seemed to align with the decreased engagement observed from mid-to-late March.
2020-05-21 - Email Geeks
Navigating Microsoft's email challenges
Drops in Microsoft email engagement and the onset of throttling can be complex issues, often stemming from a combination of factors, including shifts in user behavior, sender reputation challenges, and authentication gaps. While throttling might seem like a minor hiccup compared to hard bounces or being blacklisted (or blocklisted), its impact on timely delivery and user engagement can be significant.
Proactive monitoring, meticulous list hygiene, robust email authentication, and a commitment to delivering valuable content are your best defenses. By understanding Microsoft's unique approach to email filtering and recognizing throttling as a clear signal of underlying issues, you can implement strategies to restore your sender reputation and ensure your emails consistently reach the inbox, fostering better engagement with your audience.
