Click on the "Logout" button on any page to logout from SFUWebmail. It's important to always logout at the end of a session to prevent someone else from accessing your mail from the same web browser. You should also exit the browser if you're accessing SFUWebmail from a public workstation, as some browsers don't empty out the password field of the login page and a malicious user would be able to log back in as you.

Note: Quitting the browser is not the same as logging out from SFUWebmail. If you're accessing your mail from a public computer, do both!