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Summary

Microsoft's email rate limiting can be a significant challenge for senders, leading to delivery delays and impacting campaign effectiveness. Unlike some other mailbox providers, Microsoft's throttling mechanisms are highly dynamic, adapting based on various factors rather than adhering to rigid, published thresholds. This means that email senders might experience rate limiting intermittently, even when maintaining what they believe to be optimal sending practices. The root cause often lies in a complex interplay of sender reputation, overall volume across all Microsoft domains (including both consumer-facing and corporate Office 365 environments), and specific recipient engagement metrics.

What email marketers say

Email marketers often grapple with Microsoft's rate limiting, which can appear inconsistent and unpredictable. Many report that throttling affects sending speeds regardless of whether they adhere to perceived 'best practice' sending rates or lower volumes. A common theory suggests that Microsoft's throttling is heavily influenced by dynamic reputation assessments, rather than fixed, published thresholds. There's also a growing awareness that Microsoft considers total sending volume across all its domains (including both free webmail and Office 365) when applying limits, which can catch some senders off guard.

Marketer view

Email marketer from Email Geeks observes that Microsoft is rate limiting some client sends without a clear common pattern in terms of client or message content.

13 Nov 2020 - Email Geeks

Marketer view

Email marketer from Spotler Blog indicates that new Exchange Online recipient rate limits cap external recipients at 2,000 per user per 24 hours to manage outbound email volume.

15 Dec 2024 - Spotler

What the experts say

Email deliverability experts agree that Microsoft's rate limiting is predominantly driven by dynamic reputation signals rather than static, published thresholds. They emphasize that any perceived acceptable email sending speed is fluid and highly dependent on a sender's historical performance. Experts also point out that the absence of public data on backoff rates is intentional, as it forces senders to focus on long-term reputation building rather than trying to game a system based on fixed numbers. Understanding that all Microsoft domains are often treated as a unified ecosystem for rate limiting is a critical expert insight.

Expert view

Email expert from Email Geeks suggests that Microsoft's rate limits are highly dynamic and intrinsically tied to the sender's reputation, rather than fixed thresholds, making them difficult to predict.

13 Nov 2020 - Email Geeks

Expert view

Email expert from SpamResource states that high complaint rates or spam trap hits are common triggers for severe rate limiting by major mailbox providers like Microsoft, impacting deliverability.

10 Apr 2024 - SpamResource

What the documentation says

Microsoft's official documentation and related technical resources reveal that rate limiting is a deliberate mechanism to combat unsolicited bulk email and maintain service quality. While specific, real-time thresholds are typically not disclosed (due to their dynamic nature), the documentation does outline various limits, such as recipient rate limits per user or per tenant, and message rate limits. These limits are designed to prevent abuse and ensure fair usage of their email services, with a strong emphasis on maintaining a good sender reputation.

Technical article

Documentation from Practical 365 explains that Microsoft aims to prevent the use of Exchange Online tenants for sending spam or other unwanted email by implementing external recipient limits to control abuse.

21 Jul 2021 - Practical 365

Technical article

Documentation from University of Windsor Knowledge Base specifies that Exchange Online's message rate limit is set at 30 messages per minute per user to manage outbound email flow and maintain service quality.

10 Mar 2022 - University of Windsor - Knowledge Base

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