In the Mail app  on your Mac, select one or more mailboxes, then choose Mailbox > Export Mailbox.

Choose a folder or create a new folder, then click Choose.

Mail exports the mailboxes as .mbox packages. If you previously exported a mailbox, Mail doesn’t overwrite the existing .mbox file; it creates a new .mbox file, such as My Mailbox 3.mbox.

In the Mail app  on your Mac, choose File > Import Mailboxes.

Select a source in the list, read the information that appears below the list, then click Continue.

• If you’re importing a mailbox exported from Mail or a folder in the ~/Library/Mail/V6 folder on another Mac, select Apple Mail. To get to the Mail folder on a Mac, click the desktop to make sure you’re in the Finder, press and hold the Option key while you choose Go > Library, then click the Mail folder.

• If you’re importing messages from a Windows or UNIX computer, select “Files in mbox format,” then locate the folder containing the files.

Reorganize the imported messages, if you like.

The messages are in a new mailbox called Import in the Mail sidebar. You can drag folders and messages from the Import mailbox to new or existing mailboxes, then delete the Import mailbox.