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Microsoft's Outlook.com has been rejecting emails from legitimate senders since late January, with the problem worsening through February. Affected IP addresses receive 550 or 451 error codes citing IP reputation issues, despite those IPs showing no problems in Microsoft's own Smart Network Data Service (SNDS). Impacted senders range from public library notification systems to healthcare email providers, with critical emails like invoices, order notifications, and authentication codes being blocked. Microsoft support has been unhelpful, and the company did not respond to press inquiries. The situation highlights the collateral damage overzealous or misconfigured block list rules can cause for legitimate businesses.

Users fume over Outlook.com email 'carnage'