Sometimes the index is healthy and the service is running - but the mail file was never invited in. A PST stored in a side folder, an old archive, a data file unticked in the locations list: search honestly returns nothing for mail it does not know exists. Here is how to put the file inside the index properly.
Search can only find what the index contains, and the index only contains what it was told to include. The classic version of this problem: years of mail in an archive PST that lives in a documents folder. The file opens fine when you browse to it, so nobody suspects it - but it was never added to the Outlook profile, and Windows Search does not index PST contents as ordinary files. Outlook items are indexed through the Outlook location, for files Outlook itself knows about.
A file can exist, open, and still be invisible to search. The tick list and the profile are the two gates.
Adding a PST to the profile is what makes its contents indexable. If your archive was only ever opened by double-clicking or lives outside the profile, do this first:
The second gate is the Windows side list. Even a profile file is skipped if its location was unticked - sometimes by an over-eager cleanup tool.
An OST is the local cache of an Exchange, Microsoft 365 or IMAP account, and it is indexed as part of that account - you do not add it by hand. If the OST was recreated (a common repair step), understand what follows honestly: the new file must download the mail again and be indexed again, so search feels broken for a while on a big mailbox even though nothing is wrong. The cure is the same patience, plus the status window to prove progress.
A PST kept on a file server or NAS share is a trap with two jaws. Microsoft has never supported PST files over a network properly - they corrupt more easily there - and the Windows Search indexer does not index their contents across the network either, so the mail is both fragile and unsearchable. The honest fix is to keep the working PST on the local disk (a copy on the server is fine as a backup, not as the live file).
Watch the video to see the locations list and the profile side by side on a real machine - once you have seen the two gates, this failure never fools you again.
Watch Full VideoNo. Outlook search and the Windows index only cover data files that are part of the profile. Add the PST through File, Open and Export, Open Outlook Data File, leave it in the folder pane, and allow time for its contents to be indexed.
Because ticking a folder indexes the files in it as documents - and a PST is not indexed that way. Its contents are indexed through the Microsoft Outlook location once the file belongs to the profile. Tick Microsoft Outlook, confirm the PST is added to the profile, and then watch the item count climb.
Very likely, yes - and search is the smaller of your two problems. PST files over a network are unsupported and corruption-prone, and their contents are not indexed across the network. Move the live PST onto the PC's local disk; keep a copy on the NAS purely as a backup.