How to Create Subfolders in Apple Mail
Apple Mail comes with many features including folders and smart folders to help you organize your inboxes. If you want to choose the messages to add to your folder individually, you can create a simple mailbox and drag and drop emails to add them. If you want to create a folder that automatically sorts and organizes your emails for you, you can use the smart mailbox feature. Smart mailboxes let you apply rules, conditions and keywords to sort your email messages.
Click the Mail icon in your Dock to open Apple Mail. Alternatively, you can access Mail through your "Applications" folder.
Click the "+" sign in the bottom-left corner of the Mail application window.
Select "New Mailbox" to create a custom mailbox folder. Click the tab next to "Location" and select a destination in Mail to save your folder. To create a subfolder within an existing folder in Mail, click it as your location. Type a name for your folder into the field and click the "OK" button.
Click the desired email to select it and, while still holding your mouse, drag it to the folder. Release your mouse when you see the green "+" symbol to move your email to the folder. You can manually add as many emails as you like to a custom mailbox folder.
Select "New Smart Mailbox" to create a smart mailbox folder that automatically sorts your emails. Type a name for your smart mailbox folder into the field and then select ythe rules you want to apply. Click the "From" and "Contains" tabs to see the available rules and conditions. Click the "OK" button to create your smart mailbox. You cannot manually add emails to smart mailbox folders.