When running low on email storage you'll receive a message stating you have used 90% of your allotted mail quota. You have a few options to free up space.
A: Delete messages
B: Move messages to "local" storage (usually called "On My Mac" in Mac email).
C: Purchase more space
Moving messages is essentially the same as deleting them. Behind the scenes, your computer copies them to your computer then moves them to the Trash. Whether you delete or move messages, result is that storage is not immediately recovered because the messages go to the Trash. This can be frustrating when deleting hundreds or thousands of messages expecting to make a difference.
Space can be regained in one of two ways.
A: Log into webmail, control-click (right-click) on the Trash and select "Empty Trash." (Emptying Trash from your email app might not work properly, so that's why doing it from webmail is recommended.)
B: Wait 15 days and the Trash will automatically empty on it's own. This is the default period set by MacLab.
Related: How to Check Email Quota