You can create as many mailboxes as you like to file or organize messages you want to keep.
You can create a mailbox that's stored on your computer, one that's stored within a specific account, or a mailbox in an existing mailbox. A mailbox inside another mailbox is called a "subfolder."
If you have an IMAP account, you can store received mail on your computer or on your mail server.
Choose Mailbox > New Mailbox.
To create a mailbox within an existing mailbox, select the existing mailbox before you choose Mailbox > New Mailbox.
Use the Location pop-up menu to choose the location for the mailbox on your computer or an IMAP account (to store the mailbox on an IMAP server).
Type a name for the mailbox.
Click OK.
To create a mailbox that contains another mailbox (or subfolder), type the name for the mailbox and the subfolder separated by a forward slash ("/"). For example, if you type "Apple Mail/Apple Newsletters," a mailbox named Apple Mail will be created, with a mailbox named Apple Newsletters inside it. (Note that if you create a mailbox this way, the enclosing mailbox can only contain other mailboxes, not messages. Such mailboxes appear white in the mailboxes list, instead of blue.)
Note: Some organizations may set up mailboxes in your account that are read only. For example, a Public Folder may be used to provide information for all users of the mail system. You will not be able to create mailboxes within or save messages to these read-only mailboxes. To find out how to post messages to the mailbox, ask your mail administrator.