... and make sure you have a viable backup strategy for the whole of your PC.
EDIT: Being a little more reflective, this is space which is all too often glossed over and dismissed by suggesting the use of POP3 clients as a solution needs careful consideration.
Unless the user manages their email space, it will eventually fill the server. We all get rubbish emails which we do not delete and keep received and sent emails which have no residual value. Being diligent with house keeping and culling such emails (especially those used to share photos and documents in place of more appropriate tools) is usually more than enough to keep utilisation below 1GB.
Where a user has multiple mailboxes and / or has a lot of important historical email which needs to be retained, then users need to implement a planned archiving regime. Archiving is the removal of email from immediately on-line storage to local storage: some would suggest that POP3 is good for this, but POP3 only looks at the inbox, not any other folders which might be present. The use of POP3 with multiple clients (be they also POP3 or IMAP) invariably leads to nightmares.
Most PC email clients have intelligent tools for archiving emails to local storage folders. Archived emails will no longer be available to other clients AND it is essential that the local archives are securely backed up to external storage having a clear and tested recovery plan.
Superusers are not staff, but they do have a direct line of communication into the business in order to raise issues, concerns and feedback from the community.
