You can delete a single message in the listing pane or from the message pane (preview or popup or tab).
The deleted messages are moved to Trash folder by default. You can recover the emails in the Trash, by moving the email to the folder of your choice. The messages in Trash folder, older than 30 days will be automatically cleared at regular intervals.
However in a special scenario, all emails in the Trash folder will be automatically cleared.
When your account exceeds the allowed size, your incoming gets blocked. Before the incoming gets blocked, all the emails in the Trash are cleared and the mailbox size is recalculated. If the account size is within the allotted size, the account will not get blocked.