Hotmail email delivery failures due to connection limits are commonly linked to poor IP reputation, leading Microsoft to implement throttling. Resolving this involves a multi-faceted approach: closely monitoring IP reputation using SNDS, gradually warming up IPs, reducing the number of simultaneous SMTP connections (potentially starting below 50), pausing sending temporarily (4-12 hours), authenticating emails with SPF, DKIM, and DMARC, maintaining clean mailing lists and removing unengaged subscribers, lowering the overall sending rate, implementing retry mechanisms for throttled emails, utilizing connection pooling to optimize resource use, segmenting mailing lists to prioritize sending to engaged users, closely monitoring bounce messages for deliverability issues, experimenting to determine the optimal connection number, improving sender engagement metrics, and implementing proper queuing and handling of temporary throttling. Consulting specific error messages, adhering to ISP service agreements, and ensuring easy unsubscribe options are also crucial.
10 marketer opinions
Hotmail emails failing due to connection limits often stem from IP reputation issues leading to throttling by Microsoft. Solutions include monitoring reputation via SNDS, warming up IPs gradually, authenticating emails with SPF, DKIM, and DMARC, maintaining clean lists, reducing sending rates, implementing retry mechanisms, using connection pooling, segmenting lists for engaged users, and monitoring bounce messages. Experimentally adjusting sending rates to find the optimal connection number is also recommended.
Marketer view
Email marketer from Mailfence Blog shares advice that maintaining good sender reputation involves authenticating emails with SPF, DKIM, and DMARC. They also advise maintaining clean lists and removing unengaged subscribers to avoid spam complaints.
28 Dec 2022 - Mailfence Blog
Marketer view
Marketer from Email Geeks suggests finding the right number of connections by sending emails, adding some random wait time to spread them out. Reducing the spread until getting throttled again, then go back increasing the wait times a little. Monitor and tweak from there.
20 Apr 2025 - Email Geeks
3 expert opinions
Hotmail emails failing due to connection limits often relate to underlying issues of sender reputation, SMTP connection management, and ISP traffic control. The primary solutions involve reducing the number of concurrent SMTP connections, implementing proper queuing and retry mechanisms, improving sender engagement, authenticating email, and monitoring SNDS data.
Expert view
Expert from Word to the Wise shares insight that Microsoft's throttling is heavily influenced by sender reputation. He suggests focusing on improving engagement metrics, authenticating email, and closely monitoring SNDS data to identify and address any reputation issues.
3 Jul 2021 - Word to the Wise
Expert view
Expert from Email Geeks explains that the user should lower the number of SMTP connections in their MTA, suggesting a reduction to <50 connections to start. She recommends pausing sending for 4-12 hours and then rebuilding reputation to open more connections later. She also mentions unpublished connection limits that can cause reputation problems.
22 Dec 2024 - Email Geeks
6 technical articles
Hotmail emails failing due to connection limits are often caused by exceeding allowed concurrent connections and poor SMTP connection management. Documentation suggests reducing concurrent connections, ensuring proper connection management (closing connections properly), utilizing diagnostic tools like MXToolbox to check for blacklisting and DNS issues, and implementing SPF, DKIM, and DMARC for sender authentication. Adhering to industry standards for bulk sending, such as providing an easy unsubscribe option, is also important.
Technical article
Documentation from Postmark explains that using SPF and DKIM is the standard for email providers to know your email is actually from you, or someone who has permission to send on your behalf.
1 Aug 2023 - Postmark Guides
Technical article
Documentation from RFC Editor defines standards for SMTP connection management, emphasizing the importance of closing connections properly and avoiding excessive simultaneous connections to prevent server overload and potential blacklisting.
9 Apr 2025 - RFC Editor
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