Move or copy mailboxes
Move a mailbox: Select a mailbox, then drag it to a new location in the sidebar.
Copy a mailbox: Press and hold the Option key while dragging the mailbox.
Copy an On My Mac mailbox to the mail server: Drag the mailbox to an Exchange or IMAP account in the sidebar to copy it to the account’s mail server. This allows you to access the mailbox from any computer or device where you use that account.
Copy an Exchange or IMAP mailbox to your Mac: Drag the mailbox to an On My Mac mailbox in the sidebar. This is a quick way to create a backup copy of the mailbox.
If you use the Finder to move or copy mailboxes, the changes may not appear in Mail.