A question/issue seen frequently is how to protect Outlook data on machines used by multiple users. To summarize what can and can’t be done
Once a user is logged into the Windows account, there is no way to password protect an Outlook profile which means any user can open Outlook using any existing profile by simply going to the Control Panel <Mail> app and selecting which profile to use the next time Outlook is opened. The profile setting can be set to always prompt which profile should be used but that doesn’t prevent someone from picking whatever profile they want.
If only Exchange accounts are in use, it is possible to force the request for credentials every time Outlook is opened. This is done by going to File –> Account Settings –> Email tab –> highlight the Exchange account –> Change –> click on the <More settings button> and check the box for <Always prompt for logon credentials> as shown below
There unfortunately is no equivalent for POP3 or IMAP accounts.
You can apply a password to a PST file when it gets created but if the password is added to the Password list, it will be used automatically whenever Outlook is opened so would not prevent anyone using the computer from seeing the data. The PST password protection would prevent people from making a copy of the file and then trying to open it on another machine at which point they will need to know what the password is. Regrettably, the reality is that there are far too many password cracking programs available that will recover a password from a password protected file so the value to this approach is minimal at best.
In summary, if a machine is going to be used by multiple people then each should be set up with their own Windows account on the machine and each should have their own Outlook profile configured for use after they log in. This only works if each person logs off the machine when they are done using it, otherwise, the next person will be able to open Outlook on the machine and see the other person’s data.