<ul data-eligibleForWebStory="false"><li>Microsoft is issuing a specific error message (451 4.7.652) to indicate that the mail server has exceeded the maximum number of connections, which is related to opening too many SMTP connections.</li><li>This error message is more concrete and clear compared to generic messages, and it may become more common for bulk senders dealing with Microsoft (Hotmail/Outlook.com) domains.</li><li>The root cause of this issue could be sending too much mail too quickly, making excessive connections, not following IP warming practices, or overwhelming Microsoft's infrastructure.</li><li>To address the problem, senders are advised to slow down, reduce concurrency, follow IP warming procedures, request accommodation from Microsoft if scaling up volume, and adhere to Microsoft's sender guidelines.</li></ul>

Microsoft says: Too Many Connections

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