Why are emails to Microsoft domains throttled and how can deliverability be improved?
Michael Ko
Co-founder & CEO, Suped
Published 4 May 2025
Updated 19 Aug 2025
8 min read
Emails sent to Microsoft domains, including Outlook.com and Hotmail, often encounter unique deliverability challenges, frequently resulting in throttling. This means that Microsoft's servers intentionally slow down or temporarily block the incoming email volume from specific senders. It can be a frustrating experience, especially when you're trying to reach your audience effectively. Understanding why this happens and what steps you can take is crucial for maintaining healthy email deliverability and ensuring your messages land in the inbox.
Microsoft's email infrastructure is designed with a strong focus on protecting users from spam and phishing, which means their filtering mechanisms are particularly stringent. While this is beneficial for their users, it can be challenging for legitimate senders who need to adapt to these strict requirements. When your emails are throttled, it's often a signal that Microsoft's systems have detected something in your sending patterns or sender reputation that warrants a closer look, even if it's just a temporary measure.
The goal is always to achieve optimal inbox placement. This guide will help you unpack the common reasons for Microsoft email throttling and provide actionable strategies to improve your deliverability, helping your messages reach their intended recipients without unnecessary delays or blocks.
Why Microsoft throttles your emails
Email throttling by Microsoft is essentially a defensive mechanism. When their systems detect unusual sending behavior, a sudden surge in volume, or a potential reputation issue, they may temporarily restrict the rate at which they accept emails from a particular IP address or domain. This prevents their mailboxes from being flooded and helps to mitigate spam attacks.
The core reason behind throttling is almost always related to sender reputation. Microsoft assigns a reputation score to both your sending IP address and your domain. This score is influenced by a variety of factors, including spam complaints, bounce rates, engagement metrics (opens, clicks), and whether your email authentication records are correctly configured. A low reputation score can trigger throttling or even lead to your emails being marked as spam or blocked outright.
Unlike some other mail providers, Microsoft maintains very tight control over what enters their network, often leading to more aggressive filtering. Even if your SPF, DKIM, and DMARC records are perfectly aligned, Microsoft's proprietary filtering systems, which combine sender reputation with content analysis and user feedback, can still flag your emails.
The impact of reputation
A poor or declining sender reputation is a primary driver of throttling. If Microsoft's systems detect an increase in negative signals, such as users marking your emails as spam or a high bounce rate, they will respond by limiting the volume of email they accept from your sending infrastructure. This is a common challenge, especially for new senders or those who haven't adequately warmed up their IP and domain.
New sender challenges
New IP addresses or domains often face stricter scrutiny and throttling from the outset. This is because they lack an established sending history and a positive reputation. Building this reputation takes time and consistent adherence to best practices, as Microsoft needs to see a pattern of desired sending behavior before granting full trust.
Improving deliverability to Microsoft
Improving deliverability to Microsoft domains requires a multi-faceted approach, focusing on building and maintaining a strong sender reputation. It's not just about one factor, but a combination of technical configurations, sending practices, and engagement strategies.
Technical configurations
Firstly, ensure your email authentication protocols are perfectly set up. This includes SPF, DKIM, and DMARC. These records verify that you are who you say you are, which is fundamental for any email provider. Microsoft is particularly strict about these, and even minor misconfigurations can lead to deliverability issues. Make sure your DMARC record is configured correctly and consider a DMARC monitoring solution to actively track your authentication results and identify any failures. This helps ensure that your emails pass crucial checks like Microsoft's internal filtering and reputation checks.
A common technical issue with Microsoft is related to SPF lookups. Your SPF record may hit a DNS lookup limit, particularly if you use multiple third-party senders. This can cause authentication failures specifically for Microsoft (Outlook/Hotmail/Exchange Online). Regularly reviewing and optimizing your SPF record is essential to avoid these hidden failures and ensure compliance with their requirements.
Sending practices
Implement gradual sending, often called IP or domain warming, especially if you're a new sender or have new IPs. Don't send large volumes right away. Instead, increase your sending volume slowly over time to build a positive reputation. This helps prevent sudden volume spikes that can trigger throttling. If you’re experiencing significant delays, you may need to adjust your sending speed or rate limits with your email service provider.
Maintaining a clean email list is paramount. Regularly remove inactive, invalid, or bouncing email addresses. Sending to spam traps or disengaged users can severely damage your reputation, leading to blocklisting (or blacklisting) and throttling. Focus on sending only to engaged subscribers who genuinely want to receive your emails. This will keep your bounce rates low and improve overall inbox placement.
Leveraging Microsoft's tools and content strategies
Microsoft offers specific tools to help senders manage their deliverability: Smart Network Data Services (SNDS) and the Junk Mail Reporting Program (JMRP). SNDS provides data on your IP reputation, traffic levels, and spam complaint rates. JMRP allows you to receive copies of emails that Hotmail/Outlook users mark as junk, helping you identify problematic content or list segments.
Register for both programs to gain insight into how Microsoft views your sending practices. Regularly check your SNDS dashboard for any alerts, particularly regarding blocklisting (or blacklisting) or a change in your IP status (e.g., from green to yellow or red). This proactive monitoring is critical for identifying and addressing issues before they escalate into severe throttling or full blocks.
Content and engagement
The content of your emails plays a significant role. Avoid spammy keywords or deceptive subject lines. Ensure your email design is clean, responsive, and has a good balance of text to images. Most importantly, focus on sending highly engaging content that your subscribers value. High open rates, click-through rates, and low unsubscribe rates signal positive engagement to Microsoft, which in turn boosts your sender reputation and reduces the likelihood of throttling.
Common throttling triggers
Volume spikes: Sending a large volume of emails too quickly, especially from new IPs.
Poor sender reputation: High spam complaints, low engagement, or high bounce rates.
Authentication failures: Incorrect or missing SPF, DKIM, or DMARC records.
Spam traps: Hitting a spam trap can immediately damage your reputation.
Troubleshooting and long-term solutions
If you find your emails are frequently throttled or blocklisted (or blacklisted), it's important to act quickly. Address any technical issues first, such as SPF or DKIM errors. Then, review your sending practices: are you sending to an engaged list? Are your volumes appropriate? Sometimes, the problem might be specifically with Microsoft Outlook and Hotmail, even if other mailbox providers are fine.
For severe cases of repeated throttling or blocklisting, it may be necessary to directly contact Microsoft's deliverability support. They can provide specific insights into why your emails are being affected and offer guidance on resolution, especially for persistent issues. Remember that resolving deliverability issues with Microsoft takes time and consistent effort.
The key is to maintain a high-quality sender reputation by following best practices consistently. This includes sending only desired content to engaged recipients, maintaining proper email authentication, and proactively monitoring your sender metrics. Building trust with Microsoft's systems is a continuous process that yields long-term deliverability success.
Views from the trenches
Best practices
Actively use Microsoft's SNDS and JMRP dashboards to monitor your sender reputation and identify potential issues early.
Segment your audience and send targeted content to engaged users, minimizing complaints and maximizing positive interactions.
Rigorously clean your email lists to remove inactive addresses, bounces, and potential spam traps.
Maintain strong email authentication (SPF, DKIM, DMARC) and regularly audit your DNS records for correctness.
Common pitfalls
Ignoring low engagement metrics or high bounce rates, which can quickly degrade your sender reputation.
Sending large, untargeted email blasts without proper list segmentation or warming protocols.
Failing to implement or properly configure all necessary email authentication records.
Not reacting quickly to alerts or changes in your IP status within Microsoft's SNDS program.
Expert tips
Monitor your failure rates; keep them below one percent to avoid throttling.
Ensure your Exchange Server is updated, as outdated servers can lead to throttling and blocking.
Focus on email warming gradually building up your sending volume to establish trust with ISPs.
Consider adjusting your DMARC record to a reporting-only policy initially to avoid immediate blocks while troubleshooting.
Marketer view
Marketer from Email Geeks says they have been closely monitoring their sends to Microsoft domains to rebuild their reputation, especially since their ESP suggested the issue might be due to sending exclusively to Microsoft addresses.
April 24, 2019 - Email Geeks
Expert view
Expert from Email Geeks says senders should check their SNDS Feedback to see if they are blocklisted, as Microsoft offers a "Blocklist" feature for owned IPs. If not, their ESP should be able to provide this information.
April 24, 2019 - Email Geeks
Navigating Microsoft's email landscape
Email throttling by Microsoft domains is a common hurdle for senders, primarily driven by their rigorous reputation-based filtering systems. Rather than viewing it as an arbitrary block, consider it a signal to refine your email program and align it with best practices.
By prioritizing strong email authentication (SPF, DKIM, DMARC), maintaining a clean and engaged subscriber list, managing your sending volume strategically, and actively monitoring your sender reputation through tools like SNDS and JMRP, you can significantly improve your deliverability to Microsoft environments. Consistent effort in these areas will build trust with their filters, leading to better inbox placement and reduced throttling.
Microsoft domains requires a multi-faceted approach, focusing on building and maintaining a strong sender reputation. It's not just about one factor, but a combination of technical configurations, sending practices, and engagement strategies.
Smart Network Data Services (SNDS) and the Junk Mail Reporting Program (JMRP). SNDS provides data on your IP reputation, traffic levels, and spam complaint rates. JMRP allows you to receive copies of emails that Hotmail/Outlook users mark as junk, helping you identify problematic content or list segments.
