Why do Mailchimp email open rates for Outlook and Hotmail fluctuate, and what factors influence Microsoft deliverability?
Matthew Whittaker
Co-founder & CTO, Suped
Published 22 Jul 2025
Updated 18 Aug 2025
9 min read
Email deliverability to Microsoft's Outlook and Hotmail services can be famously unpredictable. It is not uncommon for email marketers, especially those using shared sending platforms like Mailchimp, to observe significant fluctuations in open rates for these domains, even when other mailbox providers like Gmail or Yahoo remain consistent. This variability often leads to confusion and frustration, making it difficult to pinpoint the exact cause.
The challenge lies in Microsoft's sophisticated, often opaque, filtering algorithms. Unlike some other providers that offer more detailed feedback, Microsoft's systems factor in a multitude of signals, leading to dynamic adjustments in how they treat incoming mail. A sudden drop or rise in open rates, such as experiencing low rates in one month and high rates the next, can be perplexing if the underlying factors are not understood.
To better understand these fluctuations, we need to delve into the specific mechanisms that Microsoft uses to evaluate email, the implications of sending through a shared ESP like Mailchimp, and how shifts in sender behavior or platform-wide reputation can lead to varying inbox placement and perceived open rates. Addressing these challenges requires a multifaceted approach to email security and deliverability.
Microsoft's dynamic filtering and scoring
Microsoft's filtering systems are notoriously complex, and their internal scoring can change without explicit notice. They use a combination of factors, including sender reputation, content analysis, and user engagement, to determine whether an email lands in the inbox or the junk folder. This dynamic environment can lead to significant shifts in deliverability from one period to the next, even if your sending practices seem consistent. Sometimes, emails previously marked as spam might suddenly begin landing in the inbox, leading to a spike in reported open rates. Understanding what influences Microsoft Outlook.com email deliverability is key.
A crucial aspect of Microsoft's filtering is the Spam Confidence Level (SCL) and Bulk Complaint Level (BCL) scores. These scores are assigned to incoming emails and are visible in the email headers. A low SCL score indicates a lower likelihood of being spam, while a higher BCL suggests that many recipients have marked similar emails as junk. Fluctuations in these scores can directly correlate with changes in inbox placement. It's advisable to run email deliverability tests regularly.
Example Outlook/Hotmail Header Snippettext
X-Microsoft-Antispam: BCL:2;SCL:1;
Even if your domain authentication (SPF, DKIM, DMARC) is correctly set up, Microsoft's filters consider a broader range of signals. These include recipient engagement (opens, clicks, replies), complaint rates, and whether your emails are consistently landing in the spam folder for some users. A sudden increase in positive engagement or a decrease in complaints could explain an improvement in deliverability and, consequently, open rates.
Mailchimp's shared IP reputation
Mailchimp, like many other email service providers (ESPs), uses shared IP addresses for sending emails. This means your email campaigns are sent from the same IP ranges as other Mailchimp users. While this provides a robust infrastructure for sending at scale, it also means your deliverability is intrinsically linked to the collective sending reputation of all users on those shared IPs. If another user on the same IP sends spam or engages in poor sending practices, it can negatively impact your own inbox placement, particularly with sensitive mailbox providers like Microsoft.
Mailchimp actively works to maintain a high sending reputation across its IPs, implementing measures to prevent abuse and remove problematic senders. However, fluctuations can still occur. For instance, if Mailchimp manages to clean up a problematic IP pool, or if a large number of senders improve their practices, it could lead to a sudden improvement in deliverability for everyone on that IP, reflected in higher open rates for your campaigns to Outlook and Hotmail. Conversely, if a new wave of problematic senders joins the platform, it could temporarily drag down deliverability.
While Mailchimp handles the technical authentication like SPF and DKIM by default, ensuring your own domain is properly authenticated with DMARC is crucial. This helps establish your brand's legitimacy, even when sending through a shared IP. It ensures that your domain's reputation is clearly linked to your sending, helping to mitigate the impact of other senders on Mailchimp's shared infrastructure. It is essential to ensure you are complying with the latest Outlook and Hotmail delivery requirements.
Sender reputation and engagement
Regardless of your ESP, your sender reputation is the cornerstone of deliverability. This reputation is built over time based on various signals, including your sending volume, bounce rates, complaint rates, and engagement metrics. Low engagement rates can severely impact your domain reputation over time, especially with Microsoft, which heavily prioritizes user interaction. Sudden changes in your list quality or content strategy could impact these metrics, leading to fluctuating deliverability.
User engagement, or the lack thereof, is a primary driver of deliverability. If recipients consistently open, click, and interact positively with your emails, Microsoft's filters will view you as a legitimate and valued sender. Conversely, if emails are ignored, deleted without opening, or worse, marked as spam, your reputation will suffer, leading to lower inbox placement and declining open rates. This is why improving deliverability is vital.
The content of your emails also plays a significant role. Generic, overly promotional content, or emails with many links and images without sufficient text can trigger spam filters. Personalization, relevant messaging, and a clear call to action can significantly improve engagement and signal positive intent to Microsoft. Changes in your content strategy between campaigns could account for shifts in open rates observed.
The evolving reliability of open rates
It is important to acknowledge that email open rates, especially for Microsoft domains, are no longer a perfectly reliable metric for deliverability or engagement. Modern privacy features, such as Apple Mail Privacy Protection (MPP), can artificially inflate open rates by pre-fetching email content, regardless of whether the user actually viewed the email. This means a reported 'open' may not always correspond to a genuine interaction. Consequently, a sudden spike in opens might not always signify improved inbox placement, but rather a change in how opens are being tracked or reported by the ESP or mailbox provider.
Despite this, significant fluctuations, particularly when consistent across an entire ESP like Mailchimp and specific to Hotmail or Outlook, can still indicate underlying deliverability issues. If your emails are consistently landing in the junk folder, genuine opens will be suppressed, leading to lower reported rates. Conversely, if your emails start to land in the inbox, even with inflated privacy opens, the actual visibility to users improves, which can also contribute to a higher reported rate, even if not all opens are true reads. If you are noticing Hotmail open rates declining, this could be a factor.
To get a clearer picture of your deliverability, focus on a combination of metrics beyond just open rates. Monitor click-through rates, conversions, unsubscribe rates, and, most importantly, complaint rates. Tools like Microsoft's Sender Reputation Data (SRD) program can provide some insight into how Microsoft views your sending, though it's often less detailed than Google Postmaster Tools. Regularly reviewing these diverse metrics will help paint a more accurate picture of your email program's health and provide a better understanding of why your Mailchimp email open rates for Outlook and Hotmail fluctuate.
Views from the trenches
Best practices
Maintain a clean, engaged email list by regularly removing inactive subscribers and managing bounces.
Segment your audience and send targeted, personalized content to increase relevance and engagement.
Authenticate your domain with SPF, DKIM, and DMARC to build strong sender reputation.
Monitor your deliverability metrics beyond just open rates, focusing on clicks, complaints, and conversions.
Common pitfalls
Relying solely on open rates as a measure of deliverability, especially for Microsoft domains.
Ignoring low engagement from Outlook/Hotmail users, which signals poor sender reputation.
Sending inconsistent content or high volumes to unengaged segments, triggering spam filters.
Not actively monitoring domain and IP health through postmaster tools or dedicated services.
Expert tips
Focus on content quality: engaging, personalized, and relevant content is key.
Encourage positive interactions: ask subscribers to mark as 'not junk' or add to contacts.
Use Mailchimp's reporting to identify domain-specific trends, and look for patterns in metrics.
Continuously test and adjust your sending strategy based on performance and ISP feedback.
Marketer view
Marketer from Email Geeks says that low open rates on Outlook and Hotmail often indicate emails are landing in the spam folder, so an increase in open rates suggests improved inbox placement.
2024-05-20 - Email Geeks
Expert view
Expert from Email Geeks says that Mailchimp typically handles technical compliance well, so deliverability issues are less likely to stem from basic authentication.
2024-05-20 - Email Geeks
Key takeaways
Fluctuations in Mailchimp email open rates for Outlook and Hotmail are a common challenge, stemming from a combination of Microsoft's complex filtering, the nature of shared IP sending, and the evolving reliability of open rate metrics. While a sudden improvement in open rates might be puzzling, it often indicates better inbox placement after a period where emails might have been landing in the spam folder (or blacklist).
To navigate this landscape, focus on the fundamental pillars of email deliverability. This includes maintaining excellent list hygiene, consistently sending relevant and engaging content, and monitoring a range of metrics beyond just open rates. By doing so, you can build a robust sender reputation that will stand up to the dynamic nature of Microsoft's filtering systems.
Ultimately, sustained positive engagement from your subscribers is the strongest signal to Microsoft (and other mailbox providers) that your emails are valuable. If you are experiencing issues, troubleshooting email deliverability issues can lead to better outcomes, ensuring your messages reach the intended audience and achieve their marketing goals.
