When choosing to mailbox-enable an existing object within Active Directory in the Exchange Management Console (by right-clicking "Recipient Configuration", choosing "New Mailbox" and then selecting the "Existing User" radio button), it's possible to search for multiple users within the forrest to be added which then appear in the list of objects to enable. Unlike when a single object is selected, however, when clicking the "Next" button the EMC doesn't offer the option of choosing aliases for these objects and simply displays the window with the "New" button that creates the mailboxes for these objects. In this case, does EMC automatically set the alias for these objects to be the username of each object in question - or is another format used?
Based on your description, I tested in my exchange 2010, I created 2 users in AD, screenshot below:
And I mail-enabled these 2 users in EMC, it shows:
Check the alias of these 2 users, it shows user logon names.