I spent 20 minutes trying to figure this out today. Nothing online is very complete.
Outlook 2007 (other versions act similarly), by default when you install it, auto-archives your email every 14 days. This includes deleting many calendar entries older than 6 months, which makes using Outlook for archival purposes almost impossible.
If you forgot to turn auto-archive off, here’s what to do to get that stuff back.
In the left hand “Mail Folders” pane, find the Archive Folders text. Right-click and select “Close Archive Folders.”
Now, go to Tools > Options > Other > AutoArchive. If it’s not on, turn it on for a moment. You’ll see a button next to “Move old items to:” and below that a location string, such as: C:\Users\admin\AppData\Local\Microsoft\Outlook\archive.pst. COPY this string with CTRL-C. Close this window after turning AutoArchive back off.
Next, back in the main screen, click File > Import and Export. Select “Import from another program or file.” Click Next. Select Personal Folder File (.pst), click Next. Now paste the string from the previous step into the “File to import” box. Click Next and use defaults for the remaining prompts. After a few minutes of work, all your mail and calendar items will be back where you expect them to be.