There is such a thing as off-line. And, with email that can be a good thing for multiple reasons - except when trying to send or receive of course :)
With the current incarnation of Thunderbird there is even a switch you can flip to make the email client offline, even if your PC is not. Believe it or not, that is an extremely valued feature here.
I agree strongly with this. Even on workstation I have the default to Offline and decide when to let it poll the IMAP4 servers - great for not allowing distractions or interruptions except on your own schedule, not someone else's.
With synchronized folders and offline storage it makes composing, reading, and replying easy with outgoing queued until next time connected when out and about with a portable device.
Maybe it's just me, but it is buggy to the point of being unusable every time I try it.
Example: When it is in the middle of popping up suggestions as you compose, it suddenly seems to decide you aren't in the compose window any more and interprets anything you type as a keyboard shortcut (e.g. "n" for new message "e" for archive). Since I type rather fast, this happens to me all the time.
