Microsoft's S775: The Undocumented Rate Limit That Keeps Coming Back
Since February 2026, senders have been hitting "451 4.7.650 temporarily rate limited due to IP reputation (S775)": an un...
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The Two Jobs of the Open Pixel. Only One of Them Should Survive.
CNIL's final recommendation on tracking pixels clarifies something the email industry has been avoiding for years. The t...
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Is Microsoft Still Evaluating SPF2/PRA in 2026?
The email industry believed Microsoft abandoned SPF2/PRA in 2014. Our investigation into 25,000+ daily bounces across se...
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CNIL Final Ruling on Tracking Pixels: What Changed, What We Got Right, and What It Means for Email in 2026
CNIL published its final recommendation on tracking pixels today. The consent requirement stands, but a new deliverabili...
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Deliverability Tool vs Deliverability Intelligence Platform: What's the Difference?
Seed testing, reputation monitoring, blocklist checks, ESP dashboards. Each answers one question about one data source....
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Google Postmaster Tools: The Complete Practitioner's Guide
Postmaster Tools is the most important free diagnostic instrument for Gmail deliverability. But the data arrives with de...
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The Hidden Signals: What Deferrals, Retries, and Throttling Actually Tell You
By the time bounce rates move, the problem has been building for days. Deferrals, retry patterns, and throttling behavio...
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AI for Email Deliverability: What It Can Actually Do (and What It Can't)
Most products marketed as "AI-powered deliverability" are rule engines with a modern label or content scorers checking s...
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Why Email Deliverability Monitoring Breaks at Scale
Standard deliverability monitoring assumes one ESP, one domain, and one team. Most enterprise senders operate nothing li...
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How Enterprise Email Teams Actually Diagnose Deliverability Problems
Deliverability problems are symptoms — the cause is always somewhere else. Here's the six-step diagnostic process experi...
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The Week Microsoft Broke Email for Everyone
It started with S3150. Then the rate limiting hit. Why the February 2026 Outlook crisis goes beyond a single error code,...
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Microsoft's S3150: The Most Frustrating Error Code in Email Deliverability
S3150 returns a permanent 550 error for what's often a temporary problem, with documentation that doesn't exist and a de...
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