To search the current mailbox in Mac OS X Mail from the Search Mailbox toolbar item:
- Click on the scope selector drop-down menu (the icon with the magnifying glass) to select where you want to search: Entire Message, Subject, To or From.
- Type your search term in the entry field.
Mac OS X Mail searches for matching messages as you type the term you're looking for, so you have to type only as much as is absolutely necessary.

