Mailbox storage is separate from the Vault storage included with your plan. Deleting a file from Files does not make room in Email, and deleting a message does not change the Vault usage meter.
When the mailbox approaches its limit, LightDrive sends a mailbox quota warning. Remove large messages and attachments you no longer need, then empty the mail Trash and Spam folders so their contents stop using mailbox space.
If you need to keep an attachment, save it to Files before deleting the message. That copy then uses Vault storage instead of mailbox storage.
A mailbox at its limit can stop accepting new messages. Act on a critical warning promptly and contact Support if usage does not fall after Trash has been emptied.