With previous versions of Microsoft Exchange (2007 and 2003) when we created mailbox we had to specify mailbox database for this mailbox.
In Microsoft Exchange 2010 during mailbox creating we can let exchange to choose the database for new mailbox, or we have to select option that we want to select mailbox database.
This feature is called Automatic Mailbox Distribution – this feature allow Exchange to choose mailbox database for newly created mailbox if we don’t specify mailbox database.
By default in Microsoft Exchange 2010 all mailbox databases are enabled for provisioning, but we can disable them by setting mailbox database parameters IsExcludedFromProvisioning or IsSuspendedFromProvisioning.
IsExcludedFromProvisioning – we should use this setting to permanently exlude mailbox database from automatic mailbox distribution
IsSuspendedFromProvisioning - we should use this setting to temporarily exclude mailbox database from automatic mailbox distribution.
To check provisioning settings for mailbox databases we can use:
Get-MailboxDatabase | select name, *provisioning