Hello, first thanks you for your message, it helps make Nmail better.
You had this questions because the uid.ovh bridge has a bug that allow anyone to send as anyone. It's now fixed.
Actually, you can create any identity you want but the new identity may not work.
If you want to use
[email protected], people needs a way to resolve your npub. Either your npub is inside your address either people needs to as the bridge (uid.ovh) to resolve it. So an address can resolve to only one npub.
What you have to do, go on the uid.ovh website and claim the address that you want, then you will be able to send and receive from it.