If you're using the webclient and don't send, read or open folders for more than an hour, the server assumes you are no longer working but have forgotten to log out and will log you out automatically.
If you have not yet closed your browser: log in again and then click the "Back" button in your browser until you are back in the e-mail you were working on. Now click send again and your message will be sent.
If you have closed your browser, unfortunately the information you typed has been lost.
