Thunderbird says "You are currently offline. Do you want to go online to get new messages?"
The menu item File->Work Offline is not checked.
Is there another "offline" setting?
Back story: I moved my Thunderbird profile from another user to my new account today following the instructions here. (it's down as I type). This may be a red herring.
I have been sending and receiving happily in the last few hours since - so I remain confused.
(Windows 7 Enterprise).