For security purpose, Webmail times out after a period of inactivity and automatically logs out a user even when he/she is in the middle of composing a mail.

To prevent that, one can turn on automatic email checking feature to have webmail automatically renew session. A user can go to Options --> New Mail, and set auto checking option to 'every 5 minutes'. The user then needs to keep a window open to the Inbox page.

Also, please understand that when a user leaves webmail, he/she needs to click on 'Log Out' button before closing the browser.