To see the size your folders are using, right-click your mail account and “Get Info”.
To archive email, we need to move them to a mailbox on your Mac.
To create a new mailbox, click the Plus sign and select New Mailbox. Make sure the location is “On My Mac” and call it something that makes sense to you.
Sent will probably be by far your largest mailbox. If you wish to archive other mailboxes I suggest you make a local mailbox for each and name them appropriately.
Mail dragged from your account to one of the mailboxes on your Mac will be moved and deleted from the server, freeing up space. This can take some time as we are downloading possibly thousands of messages and attachments. To watch as Mail processes use the Windows – Activity screen.
I found it difficult to work with more than around 2000 messages at a time. To get around this I made a new Smart Mailbox containing everything from the folder I was working on prior to a particular date. I was able to work with 6 months at a time.
Once the process is complete do a Get Info on your mailbox again to see how much space you’ve saved.
In order to backup these emails quit Mail, and in Finder use the Go – Go to Folder menu and type in ~/Library/Mail/V2/Mailboxes/
Backup the archive mailbox from this location.