GeeMail is a Gmail desktop client that runs on Adobe AIR and can be integrated tightly with your Gmail account without any need of configuring POP and IMAP settings.

Being in offline state one can compose the email and click the “Send” button. GeeMail is network aware; the next time it detects Internet connection, your email will be sent instantly. GeeMail work on Windows, Mac and Linux.
P.S:I think GeeMail is very useful to the users who uses dialup internet connection.