How are you accessing your email? With just a browser and the webmail UI? If you are using only the webmail, do you have more than one Comcast email address? Or do you mean accounts you have added with the mail-fetcher feature or what? When in the Compose window, look at the default address in the From: field. At the right end do you see arrowheads? Click on that to see any other accounts you have in the Comcast interface. You can use any of them to send with on a per-message basis. You might be able to do that using a desktop client like Outlook, Thunderbird or Mac Mail, etc.