GM! Thousands of emails to Outlook/Hotmail suddenly blocked - here's what happened
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
Microsoft 550s on Outlook and Hotmail often feel like broken SPF or DMARC. Plenty of times the TXT records are fine and the hit is reputation or shared-IP filtering on the ESP side. When only Microsoft destinations fail and Gmail still lands, that usually points at S3140/S3150 style blocks rather than a missing auth record.
One SPF TXT at the apex is enough. Two SPF records fail evaluation even when each one looks valid on its own. DKIM selectors need to publish, and the From domain has to align with whatever DMARC policy you set. On SendGrid and similar ESPs, PTR on the sending IPs matters as much as the domain records. Shared pools get dinged while your own DNS stays clean.
If auth checks out and reverse DNS is set, open a mitigation ticket with the exact 550 text and give it a few days. If the blocks come back when volume rises again, move Microsoft traffic onto a dedicated subdomain or IP so transactional mail stays insulated from the noisy stream.
SOLVED .. Microsoft blocking emails to outlook / hotmail / msn / live..
Curious what you all are seeing out there related to this?