I am having the same problem as those above. Just got a new Kindle, downloaded Calibre after watching your excellent tutorial, and put in my main email address rather than a Gmail address.
I do have a Gmail address but it is not the one associated with my Amazon account. Can I have more than one email address associated with my Amazon account or do I have to change it permanently to be able to use Calibre?
If this is the case, I would prefer to use Calibre without email...how would I do that?