This is really interesting stuff. You're an administrator, and you've installed a few servers running Microsoft Exchange Server 2010. You know that mailboxes are stored in mailbox databases, and, based on previous versions of Exchange, you know that you need to specify a mailbox database when you create a mailbox. One day, you go to create a mailbox and forget to specify a mailbox database. And you don't get an error. And then you ask “Where’d it go?". You eventually discover that the mailbox was created on some random mailbox database that you didn't specify yourself. You wonder what in the world just happened. Exchange administrator, meet automatic mailbox distribution.

Automatic mailbox distribution is a new feature in Exchange 2010. If you don't provide a mailbox database when you create a new mailbox on an Exchange 2010 server, Exchange picks a mailbox database for you. Now, you might see this as a really cool feature, or you might think it's pretty scary. I hope to demystify the selection process Exchange 2010 uses to select a mailbox database. Plus, I'll describe the controls you have over the process. Hopefully, armed with this information, you'll see automatic mailbox distribution as a tool you can use to simplify your life a bit. Read more on this topic at:

Where Did That New Exchange 2010 Mailbox Go?

http://technet.microsoft.com/en-us/library/ff872148.aspx