Deleting emails does not seem to reduce my storage usage

Deleting emails does not seem to reduce my storage usage

Hello - I received an 85% warning on one of my addresses (using IMAP) and when I deleted e-mails (from my Mac Mail client) and emptied trash, the % usage on the Zoho control panel stayed the same.  How should I permanently delete emails and therefore reduce my storage usage at Zoho?

thx in advance...