Summary
Microsoft rate limits email sends to manage resources, maintain reliability, and prevent abuse. Factors contributing to rate limiting include sending to both O365 and free Microsoft domains simultaneously, exceeding connection limits, poor sender reputation (due to spam complaints, sending to invalid addresses or low engagement), sending patterns resembling malicious attacks, poor list hygiene, lack of IP warming, spam-triggering content, authentication failures, volume spikes, and using shared IPs.
Key findings
- Combined Domain Sending: Sending to both O365 and free Microsoft domains at the same time can exceed overall limits.
- Resource Management: Exchange Online uses throttling to manage resources and ensure reliability.
- Attack-like Patterns: Sending patterns that resemble malicious attacks can trigger rate limiting and temporary blocks.
- Sender Reputation: Poor sender reputation, stemming from spam complaints or sending to invalid addresses, leads to rate limiting or blocking.
- Connection Limits: Exceeding the allowed number of simultaneous connections to Exchange Online triggers rate limiting.
- Poor List Hygiene: Sending emails to outdated or invalid addresses contributes to deliverability issues and rate limiting.
- Lack of IP Warming: Failing to gradually warm up a new IP address results in a poor sending reputation and rate limits.
- Content Triggers: Email content containing spam trigger words, excessive links, or poor formatting causes rate limiting.
- Authentication Issues: Failure to properly authenticate emails with SPF, DKIM, and DMARC makes emails appear suspicious, leading to rate limiting.
- Volume Spikes: Sudden increases in email sending volume trigger rate limiting, especially without proper infrastructure scaling.
- Shared IPs: Using shared IPs increases the risk of rate limiting due to the sending practices of other users on the same IP.
- High Complaint Rates: High spam complaint rates lead to rate limiting, as Microsoft identifies problematic senders through feedback loops.
- Low Engagement Rates: Low engagement rates (opens, clicks) signal unwanted email and can trigger throttling.
Key considerations
- Segment Sending: Consider segmenting sends to O365 and free Microsoft domains to avoid exceeding combined limits.
- Monitor Sending Patterns: Ensure sending patterns do not resemble malicious attacks.
- Maintain Sender Reputation: Practice good list hygiene and avoid practices that lead to spam complaints.
- Manage Connections: Control and manage the number of simultaneous connections to Exchange Online to stay within limits.
- Improve Engagement: Focus on improving email engagement rates by sending relevant content and targeting the right audience.
- Maintain List Hygiene: Regularly clean your email lists to remove invalid or inactive addresses.
- Warm Up New IPs: Gradually increase sending volume when using a new IP address to establish a positive reputation.
- Optimize Content: Avoid spam trigger words, excessive links, and poor formatting in email content.
- Implement Authentication: Ensure proper email authentication using SPF, DKIM, and DMARC.
- Scale Sending Volume: Gradually increase sending volume and ensure your infrastructure can handle the load.
- Consider Dedicated IP: If using a shared IP, consider switching to a dedicated IP for more control over your sending reputation.
- Monitor Complaint Rates: Actively monitor spam complaint rates and address any issues promptly.
What email marketers say
7 marketer opinions
Microsoft rate limits email sends for various reasons related to sender reputation, email content, and sending practices. Poor list hygiene, lack of IP warming, spam-triggering content, authentication failures, volume spikes, shared IP issues, and high complaint rates can all contribute to rate limiting.
Key opinions
- List Hygiene: Sending to outdated or invalid email addresses increases the likelihood of rate limiting.
- IP Warming: New IP addresses need to be warmed up gradually to establish a positive sending reputation; sudden large volumes can trigger rate limits.
- Content Triggers: Email content containing spam trigger words, excessive links, or poor formatting can cause rate limiting.
- Authentication: Failure to properly authenticate emails with SPF, DKIM, and DMARC can lead to deliverability issues and rate limiting.
- Volume Spikes: Sudden increases in email sending volume can trigger rate limiting, especially without proper infrastructure scaling.
- Shared IPs: Using shared IPs can increase the risk of rate limiting due to the sending practices of other users on the same IP.
- Complaint Rates: High spam complaint rates can lead to rate limiting, as Microsoft uses feedback loops to identify problematic senders.
Key considerations
- Maintain List Hygiene: Regularly clean your email lists to remove invalid or inactive addresses.
- Warm Up New IPs: Gradually increase sending volume when using a new IP address to establish a positive reputation.
- Optimize Content: Avoid spam trigger words, excessive links, and poor formatting in email content.
- Implement Authentication: Ensure proper email authentication using SPF, DKIM, and DMARC.
- Scale Sending Volume: Gradually increase sending volume and ensure your infrastructure can handle the load.
- Consider Dedicated IP: If using a shared IP, consider switching to a dedicated IP for more control over your sending reputation.
- Monitor Complaint Rates: Actively monitor spam complaint rates and address any issues promptly.
Marketer view
Email marketer from EmailDrips Blog explains that poor list hygiene practices contribute to deliverability issues. Sending emails to outdated or invalid addresses will contribute to rate limiting.
11 Nov 2022 - EmailDrips Blog
Marketer view
Email marketer from Stack Overflow notes that sudden spikes in email volume can trigger rate limiting, especially if the sending infrastructure isn't prepared for the increase. Gradual scaling is recommended.
10 Feb 2025 - Stack Overflow
What the experts say
4 expert opinions
Microsoft rate limits email sends for various reasons, including sending to both O365 and free Microsoft domains simultaneously, exceeding connection limits, high complaint rates, and low engagement rates. These factors signal potential spam activity or unwanted email, leading to throttling.
Key opinions
- Combined Domain Sending: Sending to both O365 and free Microsoft domains at the same time can exceed overall limits.
- Complaint Rates: Microsoft actively monitors and throttles senders with high complaint rates, even affecting legitimate senders.
- Connection Limits: Exceeding the allowed number of connections from a single IP address results in throttling.
- Engagement Rates: Low engagement rates (opens, clicks) signal unwanted email and can trigger throttling.
Key considerations
- Segment Sending: Consider segmenting sends to O365 and free Microsoft domains to avoid exceeding combined limits.
- Monitor Complaints: Actively monitor and address spam complaints to maintain a good sending reputation.
- Manage Connections: Limit the number of simultaneous connections from your sending IP address.
- Improve Engagement: Focus on improving email engagement rates by sending relevant content and targeting the right audience.
Expert view
Expert from Spam Resource shares that Microsoft imposes limits on the number of connections that can be made from a single IP address. Exceeding these connection limits can result in temporary throttling.
4 Sep 2022 - Spam Resource
Expert view
Expert from Spam Resource explains that Microsoft actively monitors complaint rates and throttles senders who exceed acceptable thresholds. Even legitimate senders can be affected if they generate too many complaints.
15 Aug 2022 - Spam Resource
What the documentation says
3 technical articles
Microsoft rate limits email sends to manage resources, maintain reliability in Exchange Online, and prevent malicious activities. Factors contributing to rate limiting include sending patterns resembling attacks, poor sender reputation (due to spam complaints or sending to invalid addresses), and exceeding the allowed number of simultaneous connections.
Key findings
- Resource Management: Exchange Online uses throttling to manage resources and ensure reliability.
- Attack-like Patterns: Sending patterns that resemble malicious attacks can trigger rate limiting and temporary blocks.
- Sender Reputation: Poor sender reputation, stemming from spam complaints or sending to invalid addresses, leads to rate limiting or blocking.
- Connection Limits: Exceeding the allowed number of simultaneous connections to Exchange Online triggers rate limiting.
Key considerations
- Monitor Sending Patterns: Ensure sending patterns do not resemble malicious attacks.
- Maintain Sender Reputation: Practice good list hygiene and avoid practices that lead to spam complaints.
- Manage Connections: Control and manage the number of simultaneous connections to Exchange Online to stay within limits.
Technical article
Documentation from Microsoft Learn indicates that exceeding the allowed number of simultaneous connections to Exchange Online can trigger rate limiting. Managing the number of connections is crucial for maintaining optimal sending rates.
20 Sep 2023 - Microsoft Learn
Technical article
Documentation from Microsoft Learn highlights that sender reputation plays a significant role in deliverability. Poor sender reputation, based on factors like spam complaints or sending to invalid addresses, can lead to rate limiting or blocking.
19 Apr 2025 - Microsoft Learn
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