Why is Microsoft rate limiting email sends for some customers?
Michael Ko
Co-founder & CEO, Suped
Published 3 May 2025
Updated 15 Aug 2025
7 min read
Dealing with email rate limiting from Microsoft can be a frustrating experience. One day, your emails might be flowing smoothly, and the next, you find your sends are suddenly being throttled, leading to delays and missed opportunities. This isn't just an arbitrary blockage, it's a calculated move by mailbox providers like Microsoft to protect their users and their infrastructure from potential abuse, such as spam or malware distribution.
The challenge is that these limits are rarely static or transparent. Unlike a fixed speed limit on a road, Microsoft's email sending limits are dynamic, adapting to various factors that assess the trustworthiness of a sender. This can make it difficult to pinpoint exactly why your emails are being rate limited at any given moment, especially when your sending patterns haven't noticeably changed.
It's a common scenario for many email senders, whether you're sending marketing campaigns, transactional notifications, or even internal communications. Understanding the underlying reasons for Microsoft's rate limiting, and what measures you can take, is key to maintaining good email deliverability.
The philosophy behind Microsoft's rate limiting
Microsoft's primary objective with rate limiting is to safeguard its email ecosystem. This includes protecting users from unwanted email (spam), phishing attempts, and malware, as well as ensuring the stability and performance of its services like Exchange Online. If a sending IP or domain is perceived as a threat or as potentially overburdening their systems, limits are put in place. These can manifest as temporary deferrals, message rejections, or even blocks, impacting your ability to reach recipients.
A crucial distinction Microsoft makes is between sending to internal Microsoft 365 addresses and external domains (like Hotmail.com, Outlook.com, Live.com, or other external recipients). While there are general sending limits that apply to all outgoing mail, new policies, such as the Tenant External Recipient Rate Limit (TERRL), specifically target outbound emails to external recipients. This is often an attempt to prevent compromised accounts or new tenants from being used for mass mailing spam. You can read more about Microsoft's external recipient limits.
The dynamic nature of these limits means that Microsoft's systems continuously assess the reputation of senders. This assessment is based on a complex algorithm that takes into account various signals, not just a fixed volume threshold. As a result, the throughput that Microsoft accepts can fluctuate based on your recent sending history and the perceived trustworthiness of your email program.
Understanding Microsoft's rate limiting
Purpose: Prevent spam, phishing, and ensure service stability for millions of users.
Dynamic limits: They are not fixed numbers but adjust based on your sending reputation in real time.
Recipient categories: Separate considerations for Office 365 internal recipients versus external domains (Outlook.com, Hotmail.com, etc.).
Factors influencing Microsoft's rate limits
When Microsoft decides to rate limit your sends, it's almost always a signal related to your sender reputation. This reputation is built on numerous factors, and even slight changes in your sending behavior or recipient engagement can trigger adjustments in how many emails Microsoft will accept from your IP addresses or domains. It's a complex interplay, and often not just about volume.
A primary factor is the reputation of your sending IP address and domain. If your IP address or domain has been associated with spam, even unintentionally, it can quickly find itself on an email blacklist (or blocklist). Microsoft leverages its own internal blacklists, and also monitors public ones. Being listed can significantly reduce your sending throughput, leading to a temporary rate limit due to IP reputation with Microsoft email servers. Additionally, high bounce rates, an increase in spam complaints, or a low engagement rate (opens and clicks) from your recipients can negatively impact your sender score.
Another often overlooked aspect is the sending behavior across all Microsoft domains, not just the free ones like Outlook.com. If you're sending to a mix of Microsoft 365 accounts and free personal accounts, Microsoft's systems evaluate your overall sending velocity across all these domains. For instance, you might be adhering to a self-imposed send rate for personal Hotmail accounts, but simultaneously sending a high volume to corporate Outlook accounts, inadvertently exceeding Microsoft's aggregate limits for your sending infrastructure. This combined volume can trigger rate limits even if individual streams seem acceptable. Understanding how Microsoft handles email volume limitations is essential.
Common causes of rate limiting
Poor sender reputation: Low engagement, high bounce rates, or spam complaints.
Unusual sending patterns: Sudden spikes in volume, sending to a high percentage of unknown users.
Inadequate authentication: Missing or misconfigured SPF, DKIM, or DMARC records.
Impact of rate limiting
Delayed delivery: Emails take longer to reach the inbox, affecting timeliness.
Message rejections: Emails are bounced back, sometimes with generic error codes.
Reduced inbox placement: Higher likelihood of landing in spam or junk folders.
Reputation damage: Repeated rate limiting can further degrade your sender score.
Mitigating Microsoft's rate limiting
If you're experiencing rate limiting, the good news is that there are actionable steps you can take to alleviate the issue and improve your standing with Microsoft. The core of any solution revolves around demonstrating that you are a legitimate, trustworthy sender.
First, ensure your email authentication protocols are correctly set up. This includes Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM), and Domain-based Message Authentication, Reporting, and Conformance (DMARC). These protocols verify that your emails are legitimately from your domain and haven't been tampered with. Microsoft, like other major providers, heavily relies on these to validate incoming mail. If you're unsure where to start, our guide on DMARC, SPF, and DKIM can provide a solid foundation.
Next, focus on your list hygiene. Regularly clean your email lists to remove invalid, inactive, or spam trap addresses. Sending to poor quality lists can quickly lead to high bounce rates and spam complaints, which are major red flags for Microsoft. By keeping your lists clean and ensuring you're only sending to engaged recipients, you can significantly improve your sender reputation and, by extension, your deliverability. If you are struggling with poor deliverability to Microsoft domains, this is a key step to recover from throttling, which you can learn more about in our article: Why email gets throttled by Gmail and Microsoft.
Consider your sending volume and cadence. If you've recently increased your sending volume, Microsoft might interpret this as unusual behavior. Gradually increase your sending volume (also known as warming up your IP or domain) to build trust. If you're already rate limited, try reducing your send rate and slowly increasing it again as your reputation improves. This can help you regain that "blessed grace" where your send speeds return to normal. Additionally, ensure your email content is relevant and engaging, avoiding anything that might trigger spam filters.
Views from the trenches
Best practices
Maintain a clean and engaged email list to minimize bounces and spam complaints, crucial for healthy sender reputation.
Implement and regularly check SPF, DKIM, and DMARC records for proper authentication and alignment.
Gradually increase email sending volume (warm-up) when using new IPs or domains to build trust with Microsoft.
Monitor your engagement metrics and feedback loop reports to identify potential issues early.
Segment your audience and personalize content to improve engagement and reduce unsubscribe rates.
Common pitfalls
Ignoring bounce messages and continuing to send to invalid or non-existent email addresses.
Suddenly increasing sending volume, which can trigger spam filters and rate limiting from Microsoft.
Not configuring or regularly checking email authentication records (SPF, DKIM, DMARC).
Failing to monitor spam complaints or negative feedback from recipients.
Treating all Microsoft domains (Outlook.com, Hotmail.com, Office 365) as a single entity for sending limits.
Expert tips
Always consider the aggregate sending volume across all Microsoft properties, not just individual domains.
Dynamic rate limits are reputation-based, so focus on consistent, high-quality sending rather than fixed throughputs.
Backoff rates are not publicly disclosed by major ISPs because they are constantly changing based on reputation.
If issues persist, review your email content for anything that might appear suspicious or spam-like.
Be prepared to adjust sending speeds based on real-time feedback from bounce messages and deliverability reports.
Marketer view
Marketer from Email Geeks says they're experiencing inconsistent rate limiting from Microsoft, not tied to specific clients or message content, suggesting reputation plays a role.
2020-11-13 - Email Geeks
Expert view
Expert from Email Geeks says that a potential factor could be hitting limits across all Microsoft domains, including both free mail and Office 365 addresses, combined, which might explain the varied throttling.
2020-11-13 - Email Geeks
Navigating Microsoft's email sending limits
Microsoft's rate limiting, while sometimes a hurdle, is a critical component of their spam prevention and system integrity efforts. It serves as a strong signal that adjustments are needed in your sending practices. It's rarely a punitive measure without cause, but rather an indication that your email stream requires attention to align with their expectations for legitimate senders.
By understanding that these limits are driven by a dynamic reputation assessment, and by consistently implementing email authentication, maintaining clean lists, and carefully managing your sending volume across all Microsoft properties, you can significantly improve your deliverability. Focusing on the overall health of your email program, rather than just raw sending speed, is the most effective way to navigate Microsoft's rate limits and ensure your emails consistently reach their intended recipients.
