We often encounter the need to determine how many mailboxes in a given time is in different databases or around the Microsoft Exchange Server.
Such information can be easily obtained by using the Exchange Management Shell cmdlets for Exchange 2010.
Examples of use:
Determination of the number of all mailboxes in an Exchange Organization
(Get-Mailbox -ResultSize unlimited).count
ResultSize option we use if our Exchange environment has more than 1000 mailboxes
Number of mailboxes in different databases:
Get-Mailbox -ResultSize unlimited | Group-Object -Property:Database | Select-Object Name,Count