If an email has been filed into a TonicDM project but isn't needed in the correspondence record, you can delete it.
But if you accidentally delete an email, you'll want to restore it.
Just above the list of emails in the Outlook sidebar there is a button for View Trash. Click this to show the deleted emails for this project.
When you hover the mouse over a deleted email you'll see a Restore button.
When you've restored the email, click the trash icon again to go back to the regular view of the project email.