Hotmail and t-online (Germany) are the worst E-Mail providers. They have a weird setup with weird custom white and blacklists.
My mail server is configured perfectly. However, my server is inside an IP range microsoft and telekom decided to block.
Oh... come on

Check whether your server IP has landed on any blacklist:

https://www.spamhaus.org
https://mxtoolbox.com/blacklists.as...
https://ultratools.com/tools/...

What is worse, Microsoft does not even bounce such mails, but silently drops them.

This is not to uncommon unfortunately, if your running on a personal server that range is usually flagged as dhcp even if some have the same ip consistently.

And dhcp ip’s are more often blacklisted since they often are used in bot attacks.

And even if using an ISP you could risk ending up in a range that has been abused or is a used.

One way around this is using DKIM and preferably DMARC. That allows the recipient to group you email in the certificate used ignoring any ip and I know many larger email providers do this to help avoid the ip range trap.

Running mailservers yourself is just pure pain. How many mails do you send usually? I can really recommend Postmark, i tried sendgrid, mailgun and selfhosted before that. At least if you don't send huge amounts id never go back from Postmark cause it's the only one so far that just works and never made any problems.
With sendgrid for example i also had some mails from shared ips rejected by hotmail.

In Germany, web.de is the worst provider. I'm waiting for those pour souls that subscribed to their premium package by accident decades ago to die so that this petty excuse for a provider finally can go bankrupt.

I can recommend using https://www.manitu.de/webhosting/ in Germany. 2.50€/Month for 50GB, one free domain and unlimited addresses for your domain. I use it to create things like amazon@domain.xyz to see if any service is giving away my address, so I can block them. Furthermore you can host a website if you'd want.