I run my own email server, but I use an outbound SMTP relay so that my email get delivered. It’s very very difficult to get emails from ‘new’ self-hosted mail servers into the inbox of Outlook/Hotmail users, unless you own the whole /24 IPv4 range used to send the emails, and can guarantee it won’t become anywhere close to spammy.
Since you’ve been hosting yours for a while, Microsoft might have it marked as ‘trusted’. It takes a while to get to that point though - you need to send them quite a few emails, and users need to not mark them as spam.