Troubleshooting Email Delivery to Microsoft Users

What happened

On Feb. 24, 2026 a spike of user complaints revealed a strange pattern: messages to Hotmail, Live, MSN and Outlook addresses were not arriving, while everyone else got mail just fine. SendGrid logs showed those messages were being deferred with a Microsoft response: "451 4.7.650 The mail server [redacted] has been temporarily rate limited due to IP reputation." It has been reported that the same error popped up on Microsoft’s forums within 24 hours, so the problem looked less like a single bad sender and more like a platform-level hiccup.

Impact and early detective work

This was no small annoyance. The sender moves roughly 350k emails per month, about 39k transactional (login, billing, password reset), and both dedicated IPs used for sending were hit. SendGrid’s dashboard showed a 99% sending reputation, Gmail Postmaster Tools looked normal, and SPF/DKIM/DMARC were all in place. I opened Microsoft’s SNDS and, unsurprisingly, the IPs appeared healthy from Microsoft’s telemetry too — which only deepened the mystery. Frustrating? You bet. Transactional mail is the lifeblood of many services; when it fails, customer trust can evaporate fast.

Response and what this means

A temporary work‑around was to calm users via Helpscout (emails sent there don’t route through the affected SendGrid IPs), and a support ticket with Microsoft was filed including SendGrid logs and steps taken. It has been reported that other senders saw the same behavior, which suggests a broader Microsoft throttling or misconfiguration rather than a single sender’s fault — but the timeline for a fix was unclear. The key takeaway: running transactional and bulk mail off the same limited IPs concentrates risk. When a provider’s filters flip a switch, the impact is immediate and painful. Who wouldn’t want a Plan B?

Sources: rozumem.xyz, Hacker News