...And then save the archived mail boxes to the external hard drive. That way you can always re-import them into mac mail if need be. Once you've saved the archived mail boxes to the external drive, you could create a folder in mac mail called, for example, 'Old Emails', and then just put all the old emails into there as opposed to deleting them. However, if space on your hard drive is an issue, then you may just want to delete the old emails (from mac mail on your internal hard drive).
Keep in mind if you delete the old email messages from mac mail, and then for example, have to re-setup your email account in mac mail (if issues arise, etc.) all the 'old emails' will come back. They will come back only if they exist on the email server. They won't come back if there not on the server, for example, if you told mac mail to delete the messages off the server (which can be seen in the email account preferences) or deleted them off the server directly yourself. The easiest way to know if the same old email messages exist on the mail server is to log into the your email account via webmail (where you would go to log into your email account via a web browser such as safari). Some email accounts, such as @gmail.com, give very large amounts for storage (you won't run out of space anytime soon) but some offer very little and old emails are deleted off the server to clear space for new messages.
