Troubleshooting Email Delivery to Microsoft Users

On Feb 24, 2026 we started to see a rise in user complaints saying they're not receiving our emails. They all had one in thing in common: Microsoft as their email provider. Since I'd never encountered this before, my first reaction was to check my Sendgrid logs for details. According to the logs, emails to Hotmail, Live, MSN, and Outlook email addresses were being Deferred. Emails to other email addresses were flowing normally. Digging deeper, Sendgrid showed this reason for the deferral:
451 4.7.650 The mail server [redacted] has been temporarily rate limited due to IP reputation. For e-...