You can clean up space in your mailbox by finding large attachments and saving them to your home directory (H: drive)
To do this, click "File" in the top left of your Outlook client and then the "Cleanup Tools" button. Under that, select "Mailbox Cleanup" and choose "Find items larger than" and set the number to 5000 and click "Find."
This will locate any messages larger than 5 MB and you can then sort the results by size and save the larger attached item(s) to your home directory and then delete the item from your mailbox.
Once you are done, empty your Deleted Items folder and that space will be free in your mailbox.