Once delivery is fixed, one job remains: the messages that landed in the old or wrong PST during the broken days. The good news is structural - a PST is a self-contained mailbox, so the mail is intact and portable. The only real risks are self-inflicted: importing twice and doubling everything, or deleting the file before its contents are home. This page does the rescue in the safe order.
Rule of thumb for the whole page: attach first, move second, import only on purpose, delete never (until the file is verified empty and backed up). Follow that order and this rescue cannot go wrong.
Attaching changes nothing and copies nothing - it simply shows the old file beside your current one, so you can read, search and drag mail across safely.
Attach (Open Outlook Data File) shows the old PST beside your current one: nothing is copied, nothing is merged, and you can drag across exactly what you need. Import pours the old file contents into your current file: one home for everything, but a merge you should do once, carefully. For rescuing a few weeks of stranded mail, attach wins almost every time. Import is for retiring the old file for good.
C:\Users\YourName\Documents\Outlook Files\Import the same file twice and Outlook will faithfully deliver two of everything. Attach-first working avoids the trap entirely, because dragging a selection moves those items once. If you must import, the "do not import duplicates" option is your seatbelt - and if duplicates already happened, remove them from the newer copy of the file by sorting on Created date, not by hand-picking through thousands of messages.
When the old file is empty of anything you need, right-click its name in the folder pane and choose Close - this detaches it from Outlook but leaves the file on disk, untouched. Keep that file. A PST is a complete, portable copy of that period of your mail; store it with your other backups rather than deleting it in a tidying mood. The day a client asks for a two-year-old thread, that file is the answer.
The full fix, shown live in Outlook - the account settings screens, the delivery location and the test mail arriving - is in the video above.
Watch Full VideoOld PST files sometimes carry a password set years ago - occasionally even by accident. There is no official recovery route for a forgotten PST password, and any tool promising one deserves suspicion. Try the passwords of that era first. If the file stays locked, be honest about the position: the mail inside is preserved but unreachable, which is why password-protecting a PST casually is a bad idea.
Large PST files import slowly, and an interrupted import leaves a half-merged mess to untangle. If it is running, let it run - plug in the laptop and leave Outlook alone. If you must stop, stop cleanly, then check both files carefully before re-running with "do not import duplicates" selected, or switch to the attach-and-drag method for the remainder.
No - never while Outlook is involved, and not as a rescue method at all. The two files hold different mail; overwriting destroys the newer file contents. File Explorer copying is for making a backup copy of a PST, which is a good habit. Merging mail always happens inside Outlook, with attach, drag or import.