It’s easy to find the email you need in macOS High Sierra. When you use the Mail in full screen , new messages automatically open in split view, so it’s easy to refer to other messages in your inbox as you write.
Searching your mail is fast and easy with Top Hits. Just type in the search field and the messages most relevant to your search appear in the Top Hits list.
(This how-to is based on my experiences and info on Apple’s support pages — where the images sometimes come from.)