In Microsoft Outlook email program for Mac OS X it can be surprisingly hard to find the Search function to search all email accounts and all folders. There’s a search field that is visible, but it only searches the current folder. Why Microsoft made the user interface choice to hide the ability to search is somewhat of a mystery to me, but here’s how you access all of the other search options in Outlook.
This works in outlook 2011 as well as outlook 2016 for the Mac
Folder –Searches the current folder only
Subfolders –Searches within the current folder and subfolders
All Mail boxes –Searches all email in Multiple accounts and folders
All Outlook Items – Searches all items in Outlook, including email, calendar entries, to-do entries, address book contacts, etc.