If you accidentally delete an e-mail while using the Apple Mail application on your Macintosh computer, all is not lost. The Mail application stores deleted mail in its Trash folder for a limited amount of time based on your application settings for trash deletion, or it will remain in the Trash folder until you manually delete it. After it is removed from the Trash folder, you must use a file recovery application such as Apple's Time Machine to restore the email. In order to restore the email, you must have Time Machine previously set up.
Open the Apple Mail program on your Macintosh computer and click the "Trash" tab in the application's source pane, also known as the sidebar. Alternatively, click the folder designated as the trash folder.
Select "View" on the menu bar, located at the top of your screen, and click "Show Deleted Messages" from the drop-down menu to view the erased messages in the Trash folder.
Select the message you want to remove from the deleted messages folder, and click "Messages" on the menu bar. Click "Undelete" on the drop-down menu to move the email back to the application's Inbox folder.
Open the Mail application and click the "Finder" icon on your screen's Dock. Click "Go" on the menu bar and select "Applications" on the drop-down menu.
Double-click the "Time Machine" icon to run the application. Click the "Jump Back" arrow button to view e-mail saved in Time Machine, according to David Pogue, author of "Mac OS X Leopard Edition: The Missing Manual."
Click the email message you wish to recover and click the "Restore" button at the bottom of the screen. Double-click the "On My Mac" tab in the Mail application's sidebar and open the "Time Machine" and "Recovered" folders to view the restored messages.