Deleted emails are a common accident, but luckily on a Mac they can be restored with a few simple clicks or through the use of data recovery programs such as Apple's Time Machine. As with any type of information, be sure to backup frequently to prevent permanent data loss.
Open your Mail application. Click the "Mail" icon on the dock or click the "Finder" icon, click "Go" at the top of the menu and select "Applications." Double-click "Mail" to run the application.
Click "View" at the top of the menu, followed by clicking "View Deleted Messages." Click on the "Trash" tab on the left sidebar to view your previously deleted messages. Click the message you wish to restore and click the "Messages" tab from the menu and click "Undelete" button from the drop-down menu.
Click the "Trash" icon on the sidebar. The email can easily be recovered If you deleted the email from your inbox but did not permanently delete the file from the server. Click and drag the email to the "Inbox" tab.
Time Machine Recovery
Run Time Machine. Time Machine is a program available for the Mac that allows you to restore your computer to a previously saved date. Click the "Finder" icon and click "Go" at the top of the screen.
Click the "Applications" tab from the drop-down menu and double-click "Time Machine." Click the window to change the date, as each window is a different previous restore date.
Click the "Restore" tab on the bottom of the screen when you find the date you wish to restore.