Something I think is missing from the email analogy:

We're all getting a little more confused because every server basically looks the same, so it's easy to think you could log into any of them with the same login. This is misleading, it's because all the different instances are running the same core software even though they're separate instances. It would be like if the Gmail interface was open source and people created servers on Gmail.com, Gmail.world, gbin.com and even if each of them looked the same, you could only log into the one where you created your account. And regardless you could email friends with accounts at any of them.

Hope that helps.