Donquick,
If this is a simple home network (ie one where you can easily reboot the server) then there is a quick and dirty solution: reboot of SME - this usually will fix the problem.
There must be another way but I don't know it.
What Stefano says about sharing .PST files is true. It can be done but every so often it will encounter a problem and remain "locked" because the system has set a flag that says the file is open. The more PCs that share the file, the more often this will happen (that's a rule of thumb not a provable fact - like "The more people ride bicycles, the more knees will be skinned".)
I have shared a .PSt file for years using SME and the locked file happens a couple of times year at least. A reboot of SME always fixes it. I wish I knew a better way but the solution I use is so simple (there are only two of us) I have not bothered.
The worst time we had for "locked file" with .PST was when we firrst got our ipods and had them set to sync Outlook Calendars and Contacts......which of course, aceeses the .PST file and locks it. Often it even left the file locked, especially if we disconnected the ipod by just pulling out the plug.
So, good luck with your search for a "proper" solution - post it here if you find it, please.
Cheers
Ian