Summary
Hotmail rate-limits transactional emails due to a poor IP reputation. This is because Hotmail, like other ISPs, uses IP reputation to filter spam, even for legitimate messages. Poor IP reputation arises from various factors, including a history of sending spam, listing on blocklists, low engagement with recipients, sudden volume spikes, poor list hygiene, suspicious sending patterns, and triggering spam traps. Contributing to improving IP reputation is: ensuring proper authentication (SPF, DKIM, DMARC), consistently sending high-quality emails, actively managing email lists (removing inactive subscribers), warming up IP addresses, monitoring IP reputation, and protecting web forms with CAPTCHA.
Key findings
- Reputation-Based Filtering: Outlook.com uses IP reputation to filter email, including transactional email.
- Multi-Factor Reputation: Multiple factors contribute to IP reputation, including engagement, spam complaints, list quality, and sending volume patterns.
- Authentication is Important, Not a Silver Bullet: Proper authentication is crucial, but it does not guarantee delivery if other reputation factors are negative.
- Volume Sensitivity: Sudden volume spikes can negatively impact reputation, even for legitimate senders.
Key considerations
- Monitor Reputation: Regularly monitor IP reputation using tools like Sender Score.
- Gradual Volume Increase: Warm up new IPs and gradually increase sending volume.
- Practice List Hygiene: Remove inactive or invalid addresses from your email list.
- Address Spam Traps: Ensure your emails are not triggering spam traps.
- Implement Authentication Protocols: Properly configure SPF, DKIM, and DMARC.
- Use dedicated IP: If sending a high volume of emails, consider using a dedicated IP address for reputation control.
- Implement Feedback Loops: Set up feedback loops to monitor user complaints and remove problematic subscribers.
- Protect your web forms: Protect web forms with CAPTCHA to prevent malicious submissions.
What email marketers say
12 marketer opinions
Hotmail rate limits transactional emails due to poor IP reputation, even if the emails are legitimate. Several factors contribute to this, including sudden volume spikes, poor list hygiene, sending suspicious patterns (e.g., large volumes to new recipients), and triggering spam traps. Ensuring proper email authentication (SPF, DKIM, DMARC), consistently sending high-quality emails, actively managing email lists (removing inactive subscribers), warming up IP addresses, and monitoring IP reputation are essential for improving deliverability.
Key opinions
- IP Reputation Matters: Hotmail heavily relies on IP reputation to filter emails, even transactional ones.
- Authentication is Crucial: Proper email authentication (SPF, DKIM, DMARC) is vital for establishing trust with Hotmail.
- List Hygiene is Essential: Maintaining a clean email list by removing inactive or invalid addresses improves sender reputation.
- Volume Spikes Hurt: Sudden increases in email volume can negatively impact IP reputation.
- Suspicious Patterns Trigger Throttling: Sending patterns that look suspicious (e.g., large volumes to new recipients) can lead to throttling.
- Feedback Loops Improve Delivery: Setting up feedback loops with Hotmail helps identify and remove subscribers who mark emails as spam.
- Spam Traps Harm Reputation: Triggering spam traps can severely damage IP reputation and lead to rate limiting.
Key considerations
- Monitor IP Reputation: Regularly monitor your IP reputation using tools like Sender Score.
- Gradually Increase Volume: Warm-up new IP addresses and gradually increase sending volume over time.
- Protect Web Forms: Protect web forms with CAPTCHA to prevent bots from flooding your system with requests.
- Implement DMARC: Implement DMARC to tell recipient servers what to do with messages that fail authentication checks.
- Reverse DNS: Ensure your mail server's reverse DNS is properly configured and matches the hostname.
Marketer view
Email marketer from Reddit shares that throttling by Hotmail can occur even with transactional email if your sending patterns look suspicious (e.g., large volumes to new recipients). Monitor feedback loops and adjust sending practices accordingly.
2 Apr 2024 - Reddit
Marketer view
Email marketer from Neil Patel shares that consistently sending high-quality emails, authenticating your emails with SPF, DKIM, and DMARC, and actively managing your email list (removing inactive subscribers) are key strategies to improve IP reputation.
12 Jul 2024 - Neil Patel
What the experts say
4 expert opinions
Hotmail rate-limits transactional emails due to a poor IP reputation, even for legitimate messages. This is because Hotmail, like other ISPs, uses IP reputation to filter spam. Factors contributing to this include a history of sending spam, listing on blocklists, and low engagement with recipients. Experts recommend focusing on sending only to users who want to receive mail and note that authentication alone doesn't guarantee a good reputation.
Key opinions
- IP Reputation Driven Throttling: Hotmail rate-limits or blocks emails, including transactional ones, based on the IP's reputation.
- Engagement Matters: Major mailbox providers use recipient engagement to determine the reputation of messages.
- Authentication Insufficient: Email authentication alone does not guarantee a good sender reputation or prevent rate-limiting.
Key considerations
- Target Engaged Users: Focus on sending emails only to users who want to receive them to improve engagement metrics.
- Monitor Blocklists: Regularly check if your IP is listed on any blocklists and take steps to remove it.
- Remediate Poor Reputation: Address any factors contributing to a poor IP reputation, such as spam complaints or sending to inactive addresses.
Expert view
Expert from Spam Resource explains that Hotmail, like other ISPs, uses IP reputation to filter spam, even for transactional emails. If your IP has a history of sending spam or is listed on blocklists, it may be rate-limited or blocked.
20 Mar 2025 - Spam Resource
Expert view
Expert from Email Geeks shares that the mailstream has a poor reputation with recipients so it’s being throttled.
4 Dec 2021 - Email Geeks
What the documentation says
4 technical articles
Hotmail (Outlook.com) uses IP reputation to protect users from spam, leading to rate limiting or blocking of emails from IPs with a poor reputation. This is indicated by 4xx SMTP error codes like 451, which signify temporary server-side issues, including policy restrictions like rate limiting. Using a dedicated IP address provides greater control over sender reputation. While Google's bulk email guidelines refer to bulk email, the same principles around authentication, list management and spam complaints are equally applicable to transactional email. Following best practices, including setting up SPF, DKIM and DMARC helps to ensure deliverability.
Key findings
- Reputation-Based Filtering: Outlook.com employs reputation-based filtering to protect users from spam.
- Rate Limiting Signaled by 4xx Errors: 4xx SMTP error codes, specifically 451, indicate temporary issues, including rate limiting.
- Dedicated IPs Offer Control: Using a dedicated IP address allows senders to manage their own reputation.
- Bulk Email Best Practices Apply: Best practices for bulk email, such as authentication and list management, are also relevant for transactional email.
Key considerations
- Maintain Good IP Reputation: Focus on building and maintaining a positive IP reputation to avoid rate limiting.
- Interpret SMTP Error Codes: Understand SMTP error codes to diagnose deliverability issues.
- Consider Dedicated IP: If sending a high volume of emails, consider using a dedicated IP address.
- Implement Authentication: Implement SPF, DKIM, and DMARC to authenticate your emails.
Technical article
Documentation from Microsoft Support explains that Outlook.com uses reputation to help protect customers from spam. Poor IP reputation can lead to rate limiting or blocking.
22 May 2022 - Microsoft Support
Technical article
Documentation from RFC Editor explains that 4xx SMTP codes are temporary errors, and a 451 error specifically suggests a temporary problem on the server side, often related to resource availability or policy restrictions like rate limiting.
14 Jun 2025 - RFC Editor
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