Troubleshooting Email Delivery to Microsoft Users
2 days ago
- #Email Deliverability
- #Microsoft Rate Limiting
- #Sendgrid Troubleshooting
- A sudden increase in user complaints about undelivered emails to Microsoft email addresses (Hotmail, Live, MSN, Outlook) was investigated, with Sendgrid logs showing deferrals due to IP reputation-based rate limiting.
- Microsoft's error code 451 4.7.650 indicated temporary rate limiting, despite the sender's high reputation and adherence to SPF, SKIM, and DMARC best practices, suggesting a potential issue on Microsoft's end.
- Research revealed Microsoft's hypersensitivity to spiky email traffic, leading to the hypothesis that a sudden spike in sends during a newsletter batch triggered the temporary ban.
- A Redis-backed throttling mechanism was implemented to limit sends to Microsoft users to 10 emails per minute per IP, preventing future spikes and ensuring smoother delivery.
- After escalation, Microsoft adjusted the throttling limits, and email delivery resumed within 72 hours, with no emails completely dropped but some delayed.
- The incident highlighted the need for separating transactional and bulk emails, though current volume doesn't justify a dedicated IP, and suggested Sendgrid could improve by implementing throttling at the IP level to prevent similar issues.