You want to look up old messages, not merge an entire archive into your current mailbox. An existing PST can be attached as a separate folder collection. This guide concerns an existing Outlook 2010 installation and distinguishes attaching, importing and deleting.
Start with a working copy
Use an already backed-up PST and, with Outlook closed, make a separate working copy on a local disk. Preserve the original. Opening a PST in Outlook is not guaranteed read-only access: even changing read status can modify the working file. Microsoft does not support an active PST on a network share.
Record the full path and filename of the working copy. An OST is not an equivalent archive; do not rename it to PST. For unfamiliar old data, preferably use a separate test profile without production mail flow.
Add the existing PST
- In Outlook 2010, open File, Info, Account Settings and Account Settings again.
- Select the Data Files tab and Add. This is the Outlook 2010 route documented in the archived Microsoft article.
- Select the actual existing working copy. Do not accidentally create an empty new PST with a similar name. If a file type is offered, it must match the existing archive.
- Confirm and close the settings window. The archive appears as a separate folder-list entry, possibly with a display name different from its filename.
Do not change the default data file or delivery location for this viewing task. Do not use the import wizard either: importing transfers items into a destination collection, whereas this guide keeps the archive separate.
View the correct collection
Check the added entry’s path on the Data Files tab, then open its subfolders. If messages appear missing, first check the selected folder, view filters and search scope; no search results do not prove an empty PST. Open a few known messages and compare date, sender and subject. Do not execute unknown attachments.
Do not drag items between archive and mailbox. For access or password errors, record the message and clarify permissions or the known archive password. Never repair the only original file.
Closing is not deleting
In the folder list, right-click the top-level entry for the additional archive and choose its Close command. Microsoft explains that this removes the folder-list entry, not the PST from the computer.
If Close is unavailable, stop and check whether you selected the additional archive rather than an account or default data file. Do not delete folders instead. After closing, confirm the archive entry is gone and the working file still exists at the recorded path. You can attach the same copy again when needed.