[igetspam]

It's at least partially true:

This is valid: https://mail.google.com/a/amazon.com

This is not valid: https://mail.google.com/a/thisisafakedomain.cx

All that means is that they at least used Google Apps at one time. I have a number of abandoned accounts that show as valid even though the domains are dead and the accounts are no longer used.

The same type of domain enumeration doesn't work for Drive.