A PST is an old-style storage file that keeps local copies of Outlook information. It used to be necessary before server capacity was limited and network speeds were low. There's no specific reason for PCAS users to have a PST, but one may have been created for unexpected reasons, such as subscribing to an Internet Calendar. If a PST is corrupted or deleted, it may be the only copy and therefore irrecoverably lost.
An OST is a disposable temporary file that contains the same information as a PST, but only to reduce server communications and speed up local operations. If it's corrupted or deleted, nothing is lost, as it's all on the server.
The OST or PST may become corrupt for unknowable reasons.
If you have confirmed you have no PST, then the corruption is in the OST. Reboot the computer and delete the OST.
OSTs are typically located in C:\Users\user\AppData\Local\Microsoft\Outlook
If a PST is suspected, follow the steps below.
Step 1: Reboot
This is not strictly necessary, but guarantees no other applications are accessing Outlook's data files. Some are not obvious, like search indexers, and will cause access problems you won't easily diagnose.
Step 2: Find the location of your PST
In Windows' Control Panel > Mail, click Email Accounts…
On the Data Files tab, select the suspected PST. Note its path - you will need this later.
Tip: Click Open File Location… and leave that window open.
Step 3: Fix the problem
Repair the PST with SCANPST.EXE
Run scanpst.exe, which investigates and repairs both PSTs and some OST aspects. Possible locations are listed here:
In a typical 2019 Precision Content installation, scanpst.exe it may be in
C:\Program Files\Microsoft Office\root\Office16
Enter the full path and filename of the suspected PST and click Start. A backup option is presented on the next screen. Accept all the defaults.
Restart Outlook. If everything continues to work after the next mail check, you can consider the issue resolved.
Disconnect the PST from Outlook
In a worst-case scenario, such as Outlook not even starting, it may be necessary to determine whether any PSTs exist and remove their linkage to this installation using Windows Control Panel > Mail. In the example below, the user has subscribed to an Internet Calendar which has corrupted and must be removed from Outlook, but is unable to start Outlook.
As previously, in Windows' Control Panel, click Email Accounts…
On the Data Files tab, select the suspected PST. Note its path - you will need this later. (Tip: Click Open File Location… and leave that window open.)
Click Remove to disconnect the PST from Outlook. Close these dialogs.
In the calendar, right-click the suspected calendar (already gone from the screen cap below) and choose Delete Calendar.
Shut off Outlook.
Go to the location of the suspected PST and delete or move it.
If this doesn't solve the problem, contact email@example.com.
Change send/receive group settings to prevent automatic mail check (which then hangs Outlook):
Using the Inbox Repair Tool (SCANPST.EXE) to repair Outlook Data Files:
Locating the PST
Locating the OST