Why you should not use Outlook Personal Storage (PST) files stored on a mapped or mounted drive

An Outlook Personal Storage (.pst) file is a database for storing Outlook email messages, and it is continually being updated by your Outlook client as new messages arrive and old messages are deleted or moved. Your current .pst file should be stored on the computer on which your Outlook client is running, not on a mapped or mounted drive. Outlook assumes your .pst file is stored locally, and will use operating-system-level commands that can create excessive network overhead if your .pst file is stored remotely. This can make Outlook operate very slowly, and can result in corruption of your .pst file if your network connection is intermittent or drops entirely.

For more about this issue, see Limits to using personal folders files over LAN and WAN links in the Microsoft online support documentation.