Mail opens itself a new window, without any command.

When I open Mail, a new window opens (as if I click "send a new e-mail) but I don't click anything. It opens automatically and I can't close it. The only thing I can do is to make it full screen and then minimize. Then another window opens and another and another. Finally I have my normal mail window and 5 or more at the bottom, minimized. Any idea what could it be?