Several users have reported that sometimes when they have Web Mail open for a little while, and then go to perform another operation like sending a message, they get a "Session Expired" message, and have to log in again. And if they were trying to send a composed message, unfortunately that work gets lost. Understandably, this is upsetting.
We believe we have found the cause, and it should be fixed soon. Tracking this down was due entirely to user reports in the Web Mail feedback mailbox, so thanks for letting us know.
Normally, the session timeout is controlled by your setting in Preferences / Web Mail Options, and the default is 8 hours. This is for your own protection, so that you don't accidentally stay logged in indefinitely. You can change the default setting to as high as 24 hours if you prefer. But lately a few users have been getting timed out in 30 minutes due to an issue we tracked down today from your reports. It is happening at times of high usage and folks are working on fixing it.
Be aware that even when this is working right, once you hit that 8 hours (or your setting) you will be timed out anyway. I have a planned enhancement to Web Mail so that we would save any composed message you were working on, and you could just log in again and not have your work lost. I'm making that a high priority for an upcoming release.