This happens to me when I have been logged on to comcast.net for a long time (around 6 hours) and the session expires while I'm in the middle of writing or sending an email. Instead of popping up a window to ask me to refresh my log on I just get booted at the next key stroke and I lose the email. Happens all the time.
To avoid this from happening when the session expires, do you have your Preferences set in composing (Preferences> Composing) to automatically save drafts of messages while composing? After making any Preference changes, make sure you click on SAVE.
Need Email Help? Please post the following information in your post. Do you use XfinityConnect? The Full or Lite version? Do you use an email client? Which one? (Eg; Windows Live mail, Outlook, a smartphone etc.) Which browser/version do you use? And- have you cleared your browser cache? Which operating system? XP, Vista, Windows 7, Mac OS X Details of the problem you are having.