Different kind of archives
Outlook Email Archive X is an utility for archiving emails and attachments you have received or sent using Microsoft Outlook in Mac OS X.
OEAX can archive your emails in six different ways with further sub-options.
Emails backup and restore
One of the features of OEAX is to archive emails as native Outlook file format.
The files with .eml extensions give you the possibility to restore one email or group of emails simply drop them back to Outlook at any time.
This feature will let you restore the archived email even with future versions of Office.