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Why are emails to Microsoft domains throttled and how can deliverability be improved?

Summary

Emails sent to Microsoft domains, including Outlook.com and Hotmail, are often subject to throttling or temporary blocks. This typically indicates underlying sender reputation issues rather than a specific targeting strategy of sending only to Microsoft addresses. Microsoft, like other major mailbox providers, carefully monitors incoming mail for suspicious patterns, spam complaints, and adherence to sender best practices, often using their Smart Network Data Services (SNDS) to provide insights into IP and domain health. When senders experience throttling, it suggests that Microsoft is limiting the volume of mail accepted from a particular IP or domain due to perceived risks. Understanding and addressing the root causes of these reputation issues is key to improving deliverability.

What email marketers say

Email marketers often encounter throttling when sending to Microsoft domains, leading to questions about the specific impact of targeting only these addresses. While some suspect that sending exclusively to Microsoft domains might be a factor, the consensus leans towards reputation and volume management as the primary drivers of deliverability outcomes. Marketers frequently rely on their ESP's insights and tools like Microsoft SNDS to diagnose and resolve these issues, recognizing that temporary blocks and delays are common symptoms of underlying problems rather than permanent rejections.

Marketer view

A marketer from Email Geeks indicates they have been closely monitoring their sends to Microsoft domains to rebuild their sending reputation, noting that their ESP's support believes the issue might stem from sending exclusively to Microsoft domains.

23 Apr 2025 - Email Geeks

Marketer view

A marketer from Email Geeks confirmed that their IP status in SNDS was yellow, not red, suggesting that the issue was not a hard block but rather a temporary throttle, as emails eventually completed delivery.

23 Apr 2025 - Email Geeks

What the experts say

Deliverability experts consistently state that Microsoft does not base its throttling decisions on whether a sender targets only Microsoft domains. Instead, their systems analyze broader reputation signals, including bounce rates, complaint rates, engagement, and adherence to technical standards. Temporary throttling is a common initial response to a decline in sender reputation, serving as a warning before more severe blocks or blacklisting occur. Experts advise focusing on list hygiene and overall sender health rather than the distribution of recipients across different mailbox providers.

Expert view

An expert from SpamResource.com states that email providers like Microsoft employ sophisticated algorithms to assess sender reputation based on engagement, complaints, and spam trap hits, rather than simply the destination domain.

23 Apr 2025 - SpamResource.com

Expert view

An expert from WordtotheWise.com explains that throttling is a common initial response from mailbox providers when they detect abnormal sending patterns or a decline in sender reputation, serving as a soft block to manage incoming volume.

23 Apr 2025 - WordtotheWise.com

What the documentation says

Microsoft's official documentation for senders emphasizes a strong focus on sender reputation, proper email authentication, and adherence to bulk sender guidelines. Their systems are designed to protect users from spam and phishing, which means any sending behavior that suggests lower quality or malicious intent can result in throttling, spam folder placement, or outright rejection. Documentation highlights the importance of tools like SNDS and Junk Mail Reporting Program (JMRP) for senders to understand and manage their standing with Microsoft, stressing that compliance with these requirements is paramount for consistent inbox delivery.

Technical article

Microsoft documentation states that a sender's reputation is built on various factors, including IP and domain age, sending volume, bounce rates, and spam complaint rates, all of which contribute to deliverability decisions.

23 Apr 2025 - Microsoft Documentation

Technical article

Microsoft's guidelines indicate that proper implementation of SPF, DKIM, and DMARC is crucial for authenticating emails and signaling trustworthiness, which helps prevent messages from being throttled or marked as spam.

23 Apr 2025 - Microsoft Documentation

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