diecknet

Yes, there is Exchange Server to host it yourself or Exchange Online which is the subscription based cloud service (Microsoft 365).

Ah, understood. It depends on your client OS, e.g. for Linux it should be possible. You just need to configure no nameservers in Netbird: https://docs.netbird.io/manage/dns#default-behavior-no-nameservers-configured

I use Netbird and Pi-Hole, but I don’t understand what issues you’re having. If you’re connected to Netbird, you should be able to use the Netbird DNS names to reach other devices, e.g. fileserver.netbird.cloud

Depends on if the friend is paying for the service :P