Deliverability to Microsoft famously took a dive a bit over a year ago due to random arbitrary failures within their infrastructure causing DMARC/DKIM problems which they clearly were having problems diagnosing.

Even with a six-figure email spend and weeks of troubleshooting the best response we could get from our mail provider was that they were having problems getting traction with Microsoft on the issue.

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tracker1 6 hours ago | parent [-]

Worth mentioning is that there are several email umbrellas under Microsoft... including the newer office/365, the slightly older outlook.com hosting, the old corp hosting and hotmail and sub-properties... each with different rules and services to determine spam in inconsistent ways between them.

One of my main emails is still on a "free" outlook.com hosted with a personal domain that I never shifted to paid 365. I've also got an MTA server (mailu) of my own that I've been testing with... my own email under outlook.com is literally the only one of the MS systems I can't seem to deliver to, the rest work fine. Same for google.com for that matter... kinda wild.

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oasisbob 6 hours ago | parent [-]

This issue didn't seem to discriminate. I was seeing deliverability failures to Office 365 clients as well as consumer-facing brands like hotmail and msn

	▲	tracker1 5 hours ago \| parent [-]
		Okay... I was just adding my own experience as I was able to deliver to some without issue and not the one. It's entirely possible this has changed in the past couple years since I was actively testing.