If you press on the Delete button from within any mailbox but the Trash mailbox, a highlighted (or open) email is placed in the Trash mailbox. If you highlight an email from within the Trash mailbox and press Delete, or if you use command-k to clear all email from your Trash mailbox, the email is irremediably deleted from your computer.
However, if you have access to Webmail, you can go directly to your Webmail Inbox and recover the email, provided that you have not enabled a delete-after-download option, or that it has not been more than a week (usually) since the email arrived, by which time your ISP may have deleted it from your Webmail Inbox
de