This doesnt mean entirely trashing email thats important to you. Instead, you simply want to get it out of your email program and into an archive that you can access later if necessary. In Apple Mail, just Control-click a mailbox or folder, and choose Export Mailbox. Select a destination for the archive that the program will create from the messages within that mailbox. Once Mail creates the archive, select the mailbox or the messages within that mailbox, and delete them.

This procedure is more easily done in Microsoft Outlook. Simply select the mailbox you want to archive, and drag it to the desktop; an mbox file containing the mailboxs messages will be created. Now, within Outlook, delete the contents of that mailbox.

Keep old email in another app

Speaking of archived email, it doesnt make a whole lot of sense to import thousands of messages only to read a single one, thus cluttering up the email client that you just cleaned out. Although you can read mbox files with a text editor such as Apple's TextEdit, you have a better way: Use a different email client to read your archived email.

I suggest Mozilla Thunderbird for this purpose since its free and, with the help of an add-on, able to import mbox files.