I can only confirm that out of all the email providers, Microsoft is the most tedious, non-standard and brutal when it comes to dropping email.

I'm coming from the perspective of a community site owner that pushes out email notifications to its members. Nothing spammy, all emails are opted-in and essential for the basic functioning of the community. They may be as simple as "person X liked your post". It's purely functional email. Plain text, no links, images, nothing. The entire site is non-commercial.

Further, the domain from which it is sent is well established (14 years old) and never used for sending out spam. It also has implemented all the best practices, like DMARC, SPF, the like.

It's not in the gray zone by any stretch of the imagination. It's as clean as it gets. Yet still, as random as the weather, it's always hotmail.com and live.com users complaining about email not arriving. It's not in their spam folder, it's just not there at all.

I never face this issue with any other email provider, only Microsoft.

There's no help at all regarding what triggers it, other than the standard document listing best practices I'm already following. They do have some tools to analyze issues, but guess what, only available for high volume email pushers.

There's also a public tool to check blocklists, some of which are shared between email providers, yet even when not blocked anywhere, you're still blocked at Microsoft.

So after all this self-analysis, I've learned to raise the ticket. Admittedly, occasionally they admit it was a false positive, and then lift the ban. But they never fix anything structurally, it's an issue that keeps coming back. It's maddening.

Since I'm in the mood of self-pity, I'll add to this Microsoft-specific problem by mentioning that it's hell to push out email in general for small websites.

For example, besides the functional email notifications, there's also an opt-in weekly newsletter. It's found deeply within user's personal settings, so those finding this setting and enabling it, surely want it.

Within the range of a few hundred to a few thousand subscribers, now you have a signal problem. A handful of users may no longer want the newsletter and do not unsubscribe (which is easily done from the bottom of each email). Instead, they may delete it, report it as spam, block the sender.

These are convenient to do from an email client, but the problem is that I never see their desire to unsubscribe. I'd gladly honor it. What happens instead is that in the case of low volume, just a handful of these negative signals may push all of your email into the spam folder.

Technically this is incorrect. It's not spam. The user opted in, the contents of the emails are not spammy, and the user never indicated to not want them anymore. But email providers don't care, it's all spam now.

To resolve this, I compare last activity on the site with the user's opt-in status, and auto unsubscribe everybody not recently active, regardless of whether they want this. This significantly reduces the chance of users flagging the email as unwanted in their email client. At the same time, now I have users complaining their newsletters aren't coming.

Bottom line: you can't win. With the best of intentions, fully wanting to comply with every technical and legal guidance in the book, it still does not work.