Other than a trusted outgoing IP, you can run a <10 user mailserver on a raspberry pi. The cost of setting up a trusted outgoing mailserver is also incredibly cheap (actually free if the volume is low enough).

The only things stopping businesses from doing their own mailserver is tech illiteracy, deferred liability and/or an unwillingness to maintain systems (my org has a 3rd party IT service that refuses to do anything in house, even after selling us a very decent in-house server that can definitely handle more than SSO and file storage).