Good news for Office 365 users, a future build of Windows 10 (available now in the new Windows Insider build), will restore the ability to search for email, appointments, and contacts in an Office 365 account outside of Outlook. Outlook users could initiate a search for email from the Windows shell in Windows 7 but the ability to search for Outlook items outside of Outlook was removed from Windows 8. One reason given for the change was to improve personal information security as Windows search could return results even if Outlook was closed.
Going forward, Cortana will be able to search your content in Office 365 including your emails, contacts, calendar along with files in OneDrive for Business and SharePoint. To enable this, you’ll need to connect your Office 365 work or school account to the Connected Accounts section of Cortana’s Notebook.
When you search, click the appropriate filter (email, contacts, calendar, or documents) at the top of Cortana’s search dialog (or type in calendar:, people:, or email:) to see results from your Office 365 account.
At least some filters work with the searches. Search queries found the messages I wanted to find but searching for appointments due on or message sent on a specific date wasn't always returning the expected results. Natural language dates aren't working either. It is surprisingly fast.
emails: from:firstname.lastname@example.org subject:keyword
email: from:email@example.com date:(4/28/2016)
You won’t be able to search for email using the Search field in Windows Explorer and it only searches Office 365 business accounts, not .pst files or IMAP accounts, so it's not a perfect replacement of the old feature. Because it searches the mailbox online, you can use it even if your account is not configured in an Outlook profile as results open in Outlook on the web. If you're concerned about security, disconnect the account from Cortana to prevent others from searching your mailbox, however, other people will only see basic information in the search results. They need your password to actually read the item in Outlook on the web.