It is common practice that email clients today have a search feature integrated into their functionality. This can be of great help if you want to find all the emails from one person or about a specific topic.
Some email client's search function is quite robust and will allow you to tell the application not only what to search for but where to search for it:
Other email clients, such as Mozilla Thunderbird, have a general search bar designed to search all your emails for any string of words:
Search functionality is a great feature to help you find specific emails quickly without having to look through every email.