Starting TB with/without internet connection

Is there an extension to make TB check for new mail at startup only if 
there is an existing Internet connection active?
In always connected at work but I sometimes open it at home to work on 
my mail and I keep getting the error message that it can not connect to 
the server to check the mail... duh!
I like a 4th option: "startup on line if there is already an Internet 
connection"
Eudora does it.
Thanks,