Exchange Online provides a web-based interface for searching contents of a mailbox in an organization. Administrators are able to search across a wide range of items including email messages, attachments, calendar appointments, tasks and contacts.
Previously in the Exchange Online service description there was a section that stated a it was possible to search across a maximum of 50 mailboxes at a time using the Exchange control panel or the New-MailboxSearch powershell command. That statement is correct, however, it is possible to search across more than 50 mailboxes in Exchange Online using other means.
A new service description document published on 11/17/2011 clarifies the Exchange Online capabilities for multi-mailbox search and notes the following:
The Exchange Control Panel limit can be increased up to 1000 by contacting Microsoft support. The PowerShell commandlet “Search-Mailbox” does not have a limit on mailboxes to search
View the Search-Mailbox documentation for details on how to utilize this command.
Results of multi-mailbox searches are stored in a special type of mailbox, called a discovery mailbox. A discovery mailbox has a 50 GB quota so it can store large numbers of search results. Administrators can connect Outlook to a discovery mailbox, and export the search results to a .pst file.
It is important to understand that the more mailboxes you elect to search the longer it will take for the completed results to be stored in the discovery mailbox. I would recommend that you always try to limit the number of mailboxes being searched whenever possible.
The Office 365 Service descriptions are the official documents detailing the features and functionality of the Office 365 service offerings. Information in the Office 365 documents overrides any information you find on this or any other blog or website. Always completely read and understand the service descriptions before making any decisions related to Office 365.