Why is my Microsoft FBL complaint rate suddenly higher through Mailgun?
Matthew Whittaker
Co-founder & CTO, Suped
Published 27 May 2025
Updated 19 Aug 2025
7 min read
Recently, you might have noticed an unexpected jump in your Microsoft FBL (Feedback Loop) complaint rate, especially if you're sending emails through Mailgun. It's a puzzling situation when your usual 0% complaint rate suddenly shifts, making you wonder if Microsoft has changed its FBL policies or if there's an issue with your email service provider.
This can be particularly concerning for those who rely on Mailgun's webhook reporting for complaints. Understanding what's happening behind the scenes with Microsoft's complaint reporting and how Mailgun processes this data is key to addressing any sudden spikes. It's not always a true increase in user complaints, but sometimes a change in how that data is made available or interpreted.
Understanding Microsoft FBLs and Mailgun's role
Feedback loops are essential for email senders. They provide a mechanism for internet service providers (ISPs) like Microsoft to notify senders when their recipients mark an email as spam or junk. This feedback is critical for maintaining a good sender reputation and ensuring your emails reach the inbox. Without it, you're flying blind, unable to identify and fix issues that lead to recipient dissatisfaction.
Microsoft operates its own feedback loop program, known as the Junk Mail Reporting Program (JMRPP). This program allows legitimate email senders to receive reports of spam complaints from Outlook.com users. Mailgun, as a large email service provider, is typically registered with these FBL programs and then aggregates this complaint data for its users. Your FBL complaint rate is a key metric in overall email deliverability.
A sudden jump from 0% to a noticeable complaint rate might not indicate a recent surge in user dissatisfaction. Instead, it could signal that complaint data from Microsoft's FBL is now being correctly processed and forwarded to your Mailgun account via webhooks, where it might have been missing before. This reporting change would give the appearance of an increase, even if the actual number of complaints hasn't changed drastically.
Another factor could be Microsoft's own filtering mechanisms. If your emails were previously going directly into the spam folder without recipients seeing them, they couldn't mark them as spam. If Microsoft has adjusted its filters, allowing more of your emails to land in the inbox (even if still unwanted), then recipients would finally have the opportunity to report them, leading to a visible FBL rate.
Potential causes for increased complaints
Even if the increase is due to a change in reporting, it's crucial to investigate if your actual complaint rates have risen. Several factors could contribute to a higher complaint rate, whether the data is newly visible or genuinely increasing. These often tie back to your sending practices and the quality of your email list.
When we see these spikes, our first instinct is to check for recent changes. Did you alter your email content, sending frequency, or target audience? Have you started sending to a new segment of your list that might be less engaged? Even if your emails are transactional, a sudden shift in the volume or nature of these emails could trigger more complaints if users perceive them as unsolicited or irrelevant.
Audience shifts: If you've started sending to older, less engaged segments of your list, they are more likely to mark your emails as spam.
Content changes: Any new or overly promotional content, especially for transactional sends, can increase complaints.
Volume spikes: Sending a much larger volume of emails than usual can be seen as a red flag by ISPs and blocklists.
Sub-account activity: If you're an email SaaS provider or ESP, one of your customers might have started sending poor-quality mail.
Review your email list acquisition practices. Did you recently add a significant number of contacts through a new method? Low-quality or purchased lists are notorious for generating high complaint rates because recipients never opted in or don't remember doing so.
Troubleshooting and mitigation strategies
Addressing a sudden rise in Microsoft FBL complaints requires a methodical approach. Start by confirming the nature of the increase, then take steps to mitigate any genuine issues and protect your sender reputation.
Before you act
Verify with Mailgun: Confirm with Mailgun support if there have been any recent changes to how they process or report Microsoft FBL data via webhooks.
Check SNDS: Log into Microsoft's Smart Network Data Services (SNDS) to compare the complaint rate reported there with what you see in Mailgun.
Review recent campaigns: Look at the content, sending volume, and recipient list for any emails sent just before the complaint rate spiked.
Actions to take
Improve list hygiene: Implement a strict list cleaning process. Remove unengaged subscribers and consider a re-engagement strategy for inactive contacts.
Enhance authentication: Ensure your SPF, DKIM, and DMARC records are correctly configured and aligned. This builds trust with Microsoft and other mail providers.
Monitor blocklists (or blacklists): Regularly check if your sending IP or domain is listed on any major blocklists. A listing can severely impact your deliverability and lead to higher complaints. Learn more about what happens when your domain is on an email blacklist.
Educate customers (for SaaS providers): If you're an email SaaS, proactive communication with customers about best practices can prevent them from damaging your shared reputation. Learn how to improve deliverability.
Microsoft has updated its sender requirements, which are designed to enhance security and deliverability. These changes mean that stricter adherence to email authentication standards (SPF, DKIM, DMARC) and lower spam complaint rates are now even more critical. If you were borderline compliant before, any shift in their filtering or your sending behavior could push you over the edge, leading to more complaints being registered.
Understanding Microsoft's new sender requirements
Microsoft's new requirements, which came into full effect in early 2024, are aimed at reducing spam and improving user experience. They include stricter enforcement of email authentication standards and a lower tolerance for spam complaints. If your historical complaint rate was low but not zero, the new thresholds might now flag your emails more aggressively.
Here's an example of a DMARC record that helps ensure compliance:
Regularly monitoring your sender reputation and complaint rates across all major ISPs is vital. This includes not just Microsoft but also Gmail and Yahoo. A sudden spike at one provider can be an early warning sign of broader issues, such as changes in subscriber behavior or content that is no longer resonating.
Proactive measures and content optimization
If you've identified a genuine increase in complaints, the next step is to address the root causes. This typically involves a multi-faceted approach focusing on email content, list quality, and user engagement. Remember, a high complaint rate doesn't just impact deliverability, but also signals a disconnect with your audience.
Reviewing your email content for any elements that might be perceived as spammy or irrelevant is a good starting point. Even transactional emails can generate complaints if the messaging is unclear or if they're sent too frequently. Ensure your calls to action are clear, and recipients understand why they're receiving the email.
Moreover, re-evaluating your subscription process can help. Implementing double opt-in ensures that only genuinely interested recipients are added to your list, significantly reducing the chances of spam complaints. For existing lists, segmenting based on engagement and sending less frequently to inactive users can also help. This approach protects your sender reputation and keeps your complaint rates manageable. Learn how to fix emails going to spam.
Views from the trenches
Best practices
Actively monitor all FBL data points, not just the ones you are used to seeing.
Always maintain pristine email list hygiene, regularly cleaning out unengaged subscribers.
Ensure full DMARC implementation and alignment across all sending domains.
Proactively educate all clients on best email sending practices if you operate a SaaS platform.
Common pitfalls
Assuming a sudden spike in FBL complaints means an immediate drop in deliverability.
Not cross-referencing Mailgun data with Microsoft SNDS for a complete picture.
Neglecting to investigate if changes in Microsoft's filtering are making complaints visible.
Failing to address the root cause of complaints, such as poor list quality or irrelevant content.
Expert tips
A shift from 0% FBL complaints to a visible rate might indicate improved FBL reporting from Mailgun, not necessarily increased user dissatisfaction.
Transactional email SaaS providers must actively manage client sending behavior to protect shared IP reputation.
Microsoft's updated sender requirements (early 2024) may lead to stricter filtering and more transparent complaint reporting.
High FBL rates can also stem from recipient lists that have become stale or disengaged over time.
Expert view
Expert from Email Geeks says that Microsoft has had the Junk Mail Reporting Program (JMRPP) since the mid-2000s. There might be specific data that comes through Validity that differs from JMRPP.
2024-03-10 - Email Geeks
Marketer view
Marketer from Email Geeks says that the JMRPP feedback is integrated through Mailgun, which means the platform should be forwarding these complaints.
2024-03-11 - Email Geeks
The path forward
A sudden increase in your Microsoft FBL complaint rate through Mailgun can be unsettling. However, it's often a signal that feedback data is now accurately reaching you, rather than solely indicating a sharp rise in user complaints. This shift, combined with Microsoft's evolving sender requirements, underscores the importance of robust deliverability practices.
By understanding how FBLs work, utilizing tools like Microsoft SNDS, and consistently applying email best practices – especially regarding list hygiene and content relevance – you can effectively manage your sender reputation and ensure your emails continue to land in the inbox. Remember, visibility into your complaint rates is a powerful tool for improvement, not just a cause for concern.
Mailgun support if there have been any recent changes to how they process or report Microsoft FBL data via webhooks.
Microsoft's Smart Network Data Services (SNDS) to compare the complaint rate reported there with what you see in Mailgun.
