You might not be able to send or receive any messages if the amount of storage you use takes up most or all of what's available. You need Delete Unneeded Items.
- Empty your junk folder:
In Outlook Mail Click "Junk Email" ---> Select : "Empty folder"
- Sweep unwanted email from your inbox or archive folders:
In your Inbox. Choose a single email from the sender whose messages you want to delete.
On the menu bar, choose Sweep.
- Delete large files
In the folder where you store your photos and docs, click Sort by ---> Size.
Delete attachments that are no longer needed by right-clicking the attachment and choosing "Delete":
- Delete messages and folders unneeded:
Delete messages and folders that you no longer need by selecting them and clicking the Delete button on the toolbar.