GoKawiilTroubleshooting 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-mail delivery information, see https://aka.ms/postmaster (S775) [Name=Protocol Filter Agent][AGT=PFA][MxId=redacted]
This raised even more questions. First, no emails were being delivered to Microsoft users at all. It wasn't a 'temporary rate limit'. It was more like a temporary ban. For how long? Unclear. And due to IP reputation? Sendgrid showed a 99% sending reputation. I opened Gmail Postmaster Tools and the spam rate looked normal. I was following all the best practices with SPF, SKIM, and DMARC. I also hadn't made any changes to my email setup recently.
A quick Google search for this error code led me to this: https://learn.microsoft.com/en-us/answers/questions/5786144/all-sending-ips-temporarily-rate-limited-451-4-7. Interestingly, this was posted less than 24 hours ago. Seems like I wasn't alone. This led me to believe that the problem lay with Microsoft and not me; either an outage or a misconfiguration on their end was penalizing legitimate senders. Surely, it would resolve itself...
Email deliverability is crucial for the smooth running of our business. We send ~350k emails per month of which ~39k emails are transactional in nature (login, billing, password reset, etc...). Currently, all these emails are sent from two dedicated IPs irrespective of whether the email is transactional or not. Both IPs were being rate limited.
Complaints were flowing in on Helpscout. I first instructed my CX reps to create a saved reply to calm users down. Lucky for us, emails sent via Helpscout don't go through our Sendgrid IPs.
Hi {%customer.firstName,fallback=there%},
It appears Microsoft is throttling some of our emails which is preventing the email from reaching your inbox in a timely manner.
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