In the case of Outlook and other mail programs, having an IMAP system can be painstaking in terms of limited space constraints and trying to file.
On Macs, there should be a local storage option very visible on your email program.
But with Outlook on PC the best thing to do is to make a new PST file that will hold your offline email.
Mel Bryce has lived and worked in the Kootenays for over a decade. She takes her coffee black, and still supports Nvidia graphics cards.