Troubleshooting Email Delivery to Microsoft Users
Summary
A practical case study of email delivery issues to Microsoft-hosted addresses. The author observes 451 4.7.650 deferrals due to IP reputation, investigates via SendGrid logs and SNDS, and concludes the issue may be on Microsoft's end or due to spike-driven throttling. To mitigate, they implement a Redis-backed rate limiter and batch sending, eventually restoring delivery within 72 hours, highlighting the importance of controlled sending rates and IP warming for SMBs.