Why are Microsoft complaint rates increasing and how to resolve Microsoft delivery issues?
Michael Ko
Co-founder & CEO, Suped
Published 2 Jul 2025
Updated 19 Aug 2025
11 min read
Over the past few months, many senders have noticed a significant increase in complaint rates from Microsoft domains, including Outlook.com and Hotmail. This often leads to frustrating email delivery issues, where legitimate emails either land in the junk folder or are blocked entirely. It's a challenge we've seen impact numerous businesses, causing a constant warning state for IPs, even for those with previously strong sending reputations.
This trend highlights a growing difficulty in maintaining strong deliverability to Microsoft recipients, prompting many to ask: why are these complaint rates rising, and what can be done to resolve the underlying delivery problems? It feels like we're constantly on the defensive, trying to understand an increasingly aggressive filtering system that, at times, doesn't seem to serve its own users' best interests.
Many factors contribute to this, from Microsoft's evolving filtering algorithms to changes in user behavior and the broader email ecosystem. We will explore the primary reasons behind these heightened complaint rates and provide actionable strategies to help improve your email deliverability to Microsoft domains.
Why Microsoft complaint rates are increasing
One of the primary drivers behind increasing complaint rates is Microsoft's aggressive spam filtering techniques. They have a low tolerance for what they perceive as unwanted mail, and even a slight increase in negative signals can lead to severe deliverability penalties. This means that email senders must adhere to exceptionally high standards to ensure their messages reach the inbox, rather than being shunted to the junk folder or blocked entirely. Even if you're diligently removing recipients who complain, you might not see a decrease in your complaint rates immediately.
Another significant factor is the rise of machine-generated opens, particularly from Apple Mail Privacy Protection (MPP). While MPP helps users protect their privacy, it can inflate reported open rates, making it difficult for senders to accurately gauge true recipient engagement. If you're relying solely on open rates to define your engaged audience, you might be sending to a large segment of recipients who aren't actually opening your emails. This leads to sending to less engaged users, which in turn, can contribute to higher complaint rates. It’s a tricky situation because more emails are opened on mobile devices than ever, and a portion of those are subject to MPP.
This false sense of engagement can dilute your overall sender reputation with providers like Microsoft. While machine opens don't directly influence Microsoft's internal filtering algorithms (which primarily rely on signals from their own email clients), sending to a large pool of passively engaged or unengaged recipients can still increase the likelihood of spam complaints from those who genuinely don't want your mail. This can be especially problematic when dealing with Microsoft's low tolerance for unwanted emails.
Furthermore, Microsoft has joined other major mailbox providers like Gmail and Yahoo in enforcing stricter email authentication requirements for high-volume senders. This means that proper configuration of SPF, DKIM, and DMARC is no longer optional; it's essential for ensuring your emails are trusted and delivered. Failure to meet these standards can result in emails being rejected or marked as spam, directly contributing to deliverability issues and perceived higher complaint rates. For more on this, read how to comply with Outlook's new sender requirements.
Diagnosing the problem
To effectively resolve Microsoft email delivery issues, the first step is accurate diagnosis. Microsoft employs various filtering mechanisms, and your emails could be experiencing issues due to a number of reasons, from a low IP spam reputation to specific content triggers. Understanding the difference between a block (which means your email isn't even reaching the inbox) and being routed to the junk folder (which means it's delivered, but not prominently) is crucial. Microsoft's filtering relies heavily on metrics like Spam Confidence Level (SCL) and Bulk Complaint Level (BCL), which indicate how likely your email is to be perceived as spam by their system and users. Higher SCL or BCL scores often mean your emails are being filtered.
Leveraging Microsoft's Sender Network Data Services (SNDS) and Junk Mail Reporting Program (JMRP) is essential for obtaining direct feedback from Microsoft regarding your sending reputation. SNDS provides data on your IP reputation, complaint rates, and spam trap hits, which are critical indicators of your standing with Outlook.com and Hotmail. JMRP allows you to receive copies of emails that users mark as junk, offering invaluable insights into what content or audience segments are generating complaints. This data helps you pinpoint specific issues and take corrective action.
Additionally, regularly checking your email authentication records, such as SPF, DKIM, and DMARC, is a foundational diagnostic step. Misconfigurations can severely impact deliverability. You can use an online email deliverability tester to ensure these records are correctly set up and aligned. Here's an example of how a DMARC record might look, which plays a key role in telling receiving servers how to handle emails that fail SPF or DKIM authentication:
Remember, proactive monitoring and a deep dive into these data sources are key to understanding the specific nature of your delivery challenges with Microsoft. You can also monitor for blacklists or blocklists that might be impacting your sender reputation.
Resolving Microsoft delivery issues
Once you've diagnosed the potential issues, you can implement strategies to improve your Microsoft email deliverability. The core principle is maintaining a robust sender reputation. This isn't just about avoiding spam complaints; it's about consistently sending desired emails to an engaged audience. IP reputation and recipient interaction are critical. If your domain or IP is listed on a blocklist, addressing that should be a top priority.
Best practices for email authentication
SPF: Ensure your SPF record lists all authorized sending IPs and domains.
DKIM: Implement DKIM signing for all your outgoing emails to verify sender authenticity.
DMARC: Set up a DMARC policy with reporting to monitor authentication failures and delivery outcomes. Consider moving from p=none to stronger policies like quarantine or reject incrementally. Our simple DMARC examples can help you start.
List hygiene is paramount. Regularly cleaning your email list to remove inactive, invalid, or unsubscribed addresses is one of the most effective ways to reduce bounce rates and, consequently, spam complaints. Every spam trap hit or unknown user you send to negatively impacts your sender reputation with Microsoft. Focus on sending only to engaged subscribers. This means not only removing inactive addresses but also ensuring that new sign-ups are genuinely interested in receiving your emails.
Poor list practices
Sending to unengaged users: Continuing to send to recipients who haven't opened or clicked in months, or even years. This increases the likelihood of complaints and low engagement signals.
Ignoring bounces and complaints: Failing to remove hard bounces and recipients who have marked your emails as spam. This signals to ISPs that you are not managing your list effectively.
No double opt-in: Not using a double opt-in process for new subscribers, which can lead to higher rates of invalid or disinterested sign-ups.
Improved list practices
Implement re-engagement campaigns: Attempt to re-engage inactive subscribers, and if they don't respond, remove them from your active mailing lists.
Automated list cleaning: Regularly (monthly or quarterly) remove inactive or invalid addresses, reducing bounce rates and spam complaints, as suggested by Microsoft's own guidelines.
Validate email addresses: Use real-time validation at the point of sign-up to prevent typo domains and known spam traps from entering your list.
Beyond list hygiene, ensure your email content is high quality, relevant, and engaging. Avoid characteristics commonly associated with spam, such as excessive capitalization, broken HTML, or too many images without text. Personalization and segmenting your audience based on their interests and past interactions can significantly boost engagement and reduce the likelihood of complaints. Transparent sending practices are also crucial; always make it easy for recipients to unsubscribe. This adherence to best practices directly impacts your email deliverability rates and sender reputation, especially with a vigilant provider like Microsoft.
Advanced strategies and long-term solutions
For long-term success with Microsoft deliverability, a strategic approach is necessary. Microsoft, along with other major mailbox providers, uses engagement data heavily in their filtering decisions. This means that if your emails are consistently ignored, deleted without opening, or marked as spam, your reputation will suffer, leading to higher complaint rates and reduced inbox placement. To mitigate this, consider implementing a sunset policy for inactive subscribers, even if they haven't explicitly unsubscribed. Prioritize sending to your most active segments to build positive engagement signals.
Another common issue is email throttling, where Microsoft temporarily delays or limits the number of emails you can send. This often happens if your sending volume suddenly spikes or if your reputation is borderline. To avoid this, maintain a consistent sending volume and gradually warm up new IPs or domains. If you experience throttling, it's a signal to reduce volume and focus on improving engagement and authentication before attempting to send at full capacity again. Understanding why emails to Microsoft domains are throttled is key to managing this.
For senders using shared IP addresses, particularly with platforms like Salesforce Marketing Cloud (SFMC), deliverability issues with Microsoft domains can be particularly challenging. The actions of other senders on the same IP can negatively impact your own reputation. In such cases, focusing even more intensely on your own list hygiene, segmentation, and content quality becomes critical. You might need to pull back volume or even consider dedicated IPs if the shared environment consistently causes problems. Here's a summary of common issues and their solutions:
Issue
Description
Solution
High complaint rates
Recipients are marking emails as spam.
Improve list hygiene, segment for engagement, and ensure easy unsubscribe.
Check SNDS, remove spam traps, and request delisting.
Authentication failures
SPF, DKIM, or DMARC records are misconfigured or missing.
Ensure all email authentication records are correctly set up and aligned.
Low engagement
Recipients are not opening or clicking emails.
Segment your audience, personalize content, and send relevant emails.
Ultimately, successful email deliverability to Microsoft domains requires continuous monitoring and adaptation. The landscape of email security is constantly evolving, and what worked yesterday might not work tomorrow. By staying proactive, diligently managing your lists, ensuring proper authentication, and optimizing your content for engagement, you can significantly improve your chances of consistently reaching the Microsoft inbox. It can be a slow process, sometimes requiring you to send to a very small, highly engaged portion of your list for months to rebuild trust, but the payoff in terms of deliverability is worth it.
Views from the trenches
Best practices
Maintain an aggressive list hygiene policy; remove inactive subscribers frequently to lower spam complaints and improve reputation metrics.
Use email authentication protocols (SPF, DKIM, DMARC) consistently to demonstrate legitimacy and build trust with Microsoft's filters.
Start with smaller, highly engaged segments of your audience to build a positive sending reputation before expanding volume.
Monitor Microsoft's Sender Network Data Services (SNDS) regularly for insights into your sending behavior and any red flags.
Implement a clear re-engagement strategy for inactive users, and if they don't respond, remove them to protect your sender reputation.
Common pitfalls
Relying solely on open rates, especially with Apple Mail Privacy Protection (MPP), which can inflate perceived engagement and lead to sending to unengaged users.
Ignoring complaint data from Microsoft's Junk Mail Reporting Program (JMRP) or not taking immediate action to address reported spam.
Sending to very old or unvalidated email addresses, which can lead to hitting spam traps and significantly damaging your IP reputation.
Abruptly increasing email volume without proper IP warming, which can trigger throttling or blocks from Microsoft's systems.
Neglecting to remove hard bounces promptly, signaling poor list management and potentially leading to reputation degradation.
Expert tips
Microsoft's filters are known for being aggressive; focus on sending to only those recipients who are actively opening your emails to maintain high inbox placement.
If blocks occur, reducing sending volume dramatically and being stricter on recipient selection can help resolve issues, even if it means sending to a smaller audience initially.
While Apple Mail Privacy Protection can affect reported open rates, it does not directly influence Microsoft's machine learning filters, which rely more on interactions within their own email clients.
The period following a block can make it harder to assess engagement as last opens/clicks may be pushed further into the past; focus on other engagement metrics like clicks.
Sometimes, even with all best practices, it requires a long period of consistent, high-quality sending to fully recover and regain trust with Microsoft's filtering systems.
Marketer view
Marketer from Email Geeks says they have noticed Microsoft complaints being significantly higher than before, with their IPs constantly in a warning state over the past three months, which was not previously the case.
2022-09-28 - Email Geeks
Expert view
Expert from Email Geeks says some clients are experiencing no decrease in complaint rates, even after diligently removing every recipient that complains, suggesting an aggressive filtering approach by Microsoft.
2022-09-28 - Email Geeks
Improving your deliverability to Microsoft domains
Navigating the complexities of Microsoft's email filtering can be challenging, but it is certainly manageable with a proactive and diligent approach. The recent increase in complaint rates is a clear signal that Microsoft is tightening its standards, emphasizing the need for impeccable sending practices. By understanding their aggressive filtering, diagnosing issues with tools like SNDS and JMRP, and implementing robust strategies for authentication, list hygiene, and content quality, senders can significantly improve their inbox placement.
Remember that deliverability is an ongoing effort. It requires continuous monitoring, a commitment to sending only desired mail, and a willingness to adapt your strategies based on performance data. Prioritizing genuine recipient engagement over inflated metrics like machine opens will ultimately yield better results and ensure your messages consistently reach the inboxes of your Microsoft audience.
Microsoft domains.
