I admit it. I'm really anal when it comes to allowing apps to consume hard drive space whenever and wherever they want. And Outlook is one of the worst at this. The path names make no sense to anyone but their developers and, if you have a ton of mail (like I do), this means a chunk of your system drive is gonna go bye-bye. For this reason, I force Outlook to store my mail files on a completely separate drive.
This trick is also particularly handy for those folks working on a corporate computer. Prevent Outlook from remembering your personal passwords. Then, force your data to be stored on a USB thumb drive that you take with you each day... encrypted, of course.
Okay, so here's now...
Use the registry editor to created two string keys
ForceOSTPath (for IMAP) and
ForcePSTPath (for Exchange) in the following path:
For example, if you're using Outlook 2016, the path would be:
That's really all there is to it.