{"id":5209,"date":"2026-08-11T06:48:31","date_gmt":"2026-08-11T06:48:31","guid":{"rendered":"https:\/\/www.mailercloud.com\/blog\/?p=5209"},"modified":"2026-09-08T04:58:59","modified_gmt":"2026-09-08T04:58:59","slug":"550-5-7-1-outlook","status":"publish","type":"post","link":"https:\/\/www.mailercloud.com\/blog\/550-5-7-1-outlook","title":{"rendered":"550 5.7.1 Outlook: Why Microsoft Is Blocking Your Email"},"content":{"rendered":"\n

Your campaign goes out, and Outlook and Hotmail addresses start bouncing with something like this:<\/p>\n\n\n\n

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550 5.7.1 Unfortunately, messages from [12.34.56.78] weren’t sent. Please contact your Internet service provider since part of their network is on our block list (S3150). You can also refer your provider to http:\/\/mail.live.com\/mail\/troubleshooting.aspx#errors.<\/em><\/p>\n<\/blockquote>\n\n\n\n

Microsoft is two very different systems wearing one name. Outlook.com and Hotmail are consumer mailboxes filtered by Microsoft’s own reputation engine. Microsoft 365 is tens of thousands of corporate tenants, each with an admin who can block you independently of anything Microsoft decides.<\/p>\n\n\n\n

The bounce code tells you which one you’re fighting. Get that wrong and you’ll spend a week filing delisting requests to a system that was never blocking you. This guide decodes every variant, then walks the fix in the order that works.<\/p>\n\n\n\n

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Table of Contents<\/p>\n<\/span><\/div>\n