If you know what you're looking for is the name of the sender, or perhaps part of the Subject or you're looking for a particular date, then you can choose those settings. By combining these various options you have enormous flexibility in your searches. These include serial numbers, order confirmations, problem reports, update notices and the like. Sometimes I've bought version 1 of some software, then upgraded through a couple more versions and ended up with version 4 or 5 which might have a new serial number. It didn't work - because Qualcomm sent a registration number not a serial number.
Some searches bring up far too many results.
You can narrow down your findings by searching just the results of the previous search, rather than starting again at the beginning. With your original search still open just tick the Search Results checkbox and enter a new search term. Now you'll search just those messages you've already found. This creates a new mailbox on the desktop with copies of those messages. The original found messages can be kept or deleted.