Your ISP or LAN provider may be using multiple
IP addresses or a Web Proxy server to manage your web traffic. In such a
case, the IP address seen by the WebMail server may be changing, and
the security check will log you out.
If you suspect this to be the case, try un-checking the checkbox
labelled "Restrict this session to your IP" and then login. This will
allow your WebMail session to stay alive when your IP address changes.