One way to automatically archive objects from Outlook is to use rules. To do so, first create a folder on your local computer (e.g. Archived). Then, from the Tools menu, select Rules. Create a rule by clicking on the plus sign (let’s just call it Archiving) and then click Date Received in the “When a new message arrives: section. Configure the middle field as “is Greater than or equal to” and then configure the number of days (e.g. 90 or 180). In the “Do the following:” section, choose “Move message” and then choose the archive folder you created in the previous step. Finally, check the box for Enabled and you’ve got a pretty simple automated rule to move messages off your Exchange server and onto your local computer. To move existing mail, click Rules from the Message menu and then click on the name of the rule.
Once done, click on the Outlook menu and then click on Preferences. Click General and then in the Folder list section, check the box for “Hide On My Computer folders”.
The only problem with this method is that mail is just getting dumped into a folder on your local computer. Mail is searchable, but not stored in a .pst file as with the old school keep it on a mapped drive method frequently used with Outlook for Windows. These folders can be exported into .mbox files by dragging a folder to the desktop. The maximum size of the file is 2.15 gigs. The .mbox file can then be imported using the Import option under the File menu and then clicking on the .mbox file in question.
Another, more automated and old fashioned way to archive mail is to use Outlook Email Archive X, a tool that takes care of much of the tasks you just did. This tool, from softhing.com, is available at http://www.softhing.com/oeax.html. I like Outlook Email Archive X because I can drop emails back into outlook because they’re stored in .eml files. These are also indexed using Spotlight. To install Outlook Email Archive X, extract the dmg from the downloader. Then drag the OEAX folder to the /Applications folder.
Modern Exchange and Office 365 instances provide archival options, as do tools such as GFI. Outlook is pretty much AppleScriptable. So you should be able to automate this stuff if you so choose if you don’t have access to any of the other tools. Good luck!