I don't disagree, but what are your options when a giant portion of the population uses O365 and outlook.com? How are you going to get them to stop using it, so that legit emails from user-owned servers are not silently rejected?

Most of them wouldn't even care probably...

I use a sending service. I would prefer not having to, but at least all inbound email goes directly to my server.

I've also been running my own server for over 20 years, but do note that the big players may block their senders from delivering mail to your server. This happened to me with GMail around 6-7 years ago, until I configured DMARC and/or DKIM and/or SPF to their liking. As far as I know, they don't do a press release when they update their rules.

I have a perfect score in google postmaster tools on dkim, spf and 0 spam reports. They still send all emails to spam. Same experience for my friend running an indie festival. These days you pretty much have to alert gmail users by some other means (like if signing up on a website) that they have to check spam and whitelist you since it appears gmail would rather block legit emails than allow a single possible spam through.

It depends on the nature of the transaction. If the user needs the email more than you do, you just remind them Outlook deletes your messages, and they'll find a way, and in the long term usage of Outlook will decrease.

>I've been running my own mail server for over 20 years and outlook.com is literally the only domain I have had problems with

What about tencent.com, foxmail.com, or qq.com?

I still run my own server and do not use a relay service and do host on a cheap (free actually, OCI) VPS and other cheap VPS's before that since the 1990's. In that time only one big email service blocked my IP by a large block range, twice...Microsoft but by filing a support request here https://olcsupport.office.com/ both times, they resolved in in less than 2 days. The second incident was just in the last several days so the request does work.