[EX2010] Search-Mailbox

The Search-Mailbox cmdlet returns up to 10000 results per mailbox if a search query is specified. To return more than 10000 results, use the New-MailboxSearch cmdlet or the In-Place eDiscovery & Hold console in the Exchange Administration Center. 

From <http://community.office365.com/en-us/f/158/t/239292.aspx>