Troubleshooting Email Delivery Failures to Microsoft Users

Starting on February 24, 2026, emails began getting Deferred only for Hotmail, Live, MSN, and Outlook recipients, and Sendgrid logs showed a 451 4.7.650 error along with a temporary rate limit message based on IP reputation.

At first it looked like a problem on Microsoft's side, and Gmail Postmaster Tools, SPF/DKIM/DMARC, Sendgrid reputation, and even Microsoft's SNDS were checked, but nothing unusual was found. Even after submitting logs and evidence through the Microsoft support portal, the initial response was essentially "the IP is fine."

While reexamining the cause, the hypothesis emerged that this wasn't simply a reputation issue but could be caused by spiky traffic concentrated in a short period. Indeed, right before the weekly newsletter send, there were 53 sends per minute to Microsoft recipients, and compared to the pattern of the previous few months, this point in time was the most suspicious.

As a response, a Redis-based rate limiter was implemented directly.

• RedisRateLimiter for sliding-window allow/wait decisions
• Created IPPoolRateLimiter to apply limits per IP pool
• Throttled Microsoft-family addresses to 10 per minute per IP
• When the limit is hit, wait for retry_after before proceeding

Alongside this measure, another appeal was sent to Microsoft, and a human response later came back saying "your IP throttling limitation has been set to a more appropriate level." Within a few hours after that, Sendgrid's sending to Microsoft recipients returned to normal.

In the end, only some emails were delayed for about 72 hours, with no loss of mail, and sending to Microsoft recipients has since been kept from exceeding 10 per minute per IP.

The key takeaways are as follows.

• Microsoft can temporarily and strongly throttle certain sending IPs.
• Even when reputation figures appear normal, a sudden spike in traffic itself can trigger problems.
• For bulk sending systems, per-recipient-provider rate limiting is essentially necessary.
• If ESPs like Sendgrid throttled sends to Microsoft more conservatively, such incidents could be reduced.