From my limited experience, Hotmail and Outlook love blacklisting IPs. If you are on Basic/Essentials plan on Sendgrid, your emails get sent from the same IP as potentially hundreds of other sites… If one of those sends bad emails and gets blacklisted, it affects everyone at once on that IP.

If I were in your shoes, I’d evaluate getting a dedicated IP with Sendgrid - Which unfortunately requires a Pro plan…

I’m sure others will have other possible explanations. Just my 2 cents.

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