From a different Lemmy post I wrote a month ago:

I also host my own mail and there’s been little issues.

Microsoft is a pain in the ass if you’re in an IP space they don’t like like DigitalOcean. Which is ironic because they have the worst spam filter by far in the industry.

If you want to get through to everyone you will have to:

• Use a “good” TLD ( not .to, not .xyz, …)
• Don’t use cloud platforms that are regularily used for spam (mostly DigitalOcean)
• Use SPF
• Use DMARC
• Use DKIM
• Use a PTR record
• Don’t make an open relay by accident
• Use proper ports and certificates
• Register an abuse account at the big players (Google, Microsoft, …)
• Don’t use an dynamic IP
• Keep it up to date
• Minimize downtime

I can’t recommend mailcow enough, it makes setting up a mail server a breeze.

https://github.com/mailcow/mailcow-dockerized

Use the MXToolbox to verify your server(s).

https://mxtoolbox.com/diagnostic.aspx