I am unable to "Quit Mail" and therefore, unable to shut down the computer.

When I try to close the program, I get the message, "Cannot send message using the server verizon.net. The server "outgoing.verizon.net" did not recognize the following recipients:" and then there is a list of over 400 names. I am sure the length of the Bcc list is why the message did not go out. However, now I am stuck in a loop. The red light button in the top left corner of the window is grayed out, so not usable. When I try to "Quit Mail" that message pops up. When I try to shut down the entire computer, I am told to first close the mail program?D!