Several methods for categorizing email messages are provided. You can use these along with the Search feature to quickly locate and retrieve messages. Methods you can use are:
Filters. In Preferences>Mail>Filters, you can create rules for classifying inbound mail according to items in the subject line, sender, recipient, or other characteristics.
Folders. You can create folders to organize your messages and you can move mail from one folder to another. For example, taking a mail out of a "To Do" folder and moving it to a "Done" folder.
Tags. You can use tags as labels on email messages and then use the Search feature to show all mail with a certain tag.
Conversations. You can have messages grouped by conversation. This built-in feature automatically groups all sent and received email messages with the same subject into a single conversation.