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GM! Thousands of emails to Outlook/Hotmail suddenly blocked - here's what happened

by Will Rasol
https://support.sendgrid.com/hc/en-us/articles/38465017420955-Troubleshooting-Microsoft-Delivery-Issues-550-5-7-1-S3140-S3150-Blocks

GM.. Updating my builds & noticed, I've had 1000's of emails not being delivered to Outlook Hotmail & other Microsoft domains ALL THE SUDDEN.. Nasty 550 blocks. Thought it was me, until I saw this article from Sendgrid - Might want to read before losing sleep over SPF's and DMARC

Will Rasol's avatarWill Rasol
submitted this linkon February 25, 2026
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    SOLVED .. Microsoft blocking emails to outlook / hotmail / msn / live..

    1. Email support the exact 550 error - ask for mitigation
    2. Double Triple Quadruple check SPF / DKIM / DMARC - https://mxtoolbox.com has been good to me.
    3. Double-check verification DNS records (usually TXT records) for 3rd-Party senders if you have any
    4. If you use Sendgrid (which I did in my case).. make sure "Reverse DNS" is configured. Under settings I believe. This I think was THE #1 problem
    5. Wait 3 days. Now working 😀

    Curious what you all are seeing out there related to this?

    Will Rasol's avatar Will Rasol
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    6 months ago
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