So who stores the login information? This is fundamentally the question here.
If you store it centrally you only need to ask for username/password combo.
But then someone needs to store it at a central location for everyone to check against.
If it’s not centralized than the user needs to provide it
Email has a hidden trick up its sleeve and that’s the domain name. In order for an email to be valid, the domain name must contain email info on its DNS records. There’s where you can imply knowledge about where the email/message is to go.
But here in lemmy, my email is just Gmail. There’s no way to find the information on where authentication could be located. Which brings me back to the top of centralization vs decentralization.