If you occasionally deleted important email, do not start retyping it before you try out a couple of simple recovery methods. A problem of accident deleting is quite widespread, so different email client programs have their own features for deleted email recovery. Here is a short guide for you.
First of all you should look into Trash folder. Most of clients have Trash catalogue where all deleted emails are stored. Trash contains messages that were deleted from inbox or Sent folders. If you were in the process of composing an email and simply closed the program or web interface page, reopen it and go to Drafts folder. For instance, Gmail saves drafts automatically, so your message most likely will stay in Drafts.
If neither Trash nor Drafts contains your deleted email, then you can try a data recovery software. These tools are efficient for desktop mail client programs such as Mircosoft Outlook. A program that is specialized in recovering MS Outlook emails is Data Recovery Wizard. As an alternative you can try ParetoLogic Data Recovery tool that is able to recover deleted emails on various operating systems.
Remember that from time to time it is useful to backup important emails for security purposes. You can convert your emails to Doc and them view them in your Microsoft Word or any other doc viewer.