Tkinter: How to get frame in canvas window to expand to the size of the canvas?

So I've been using the canvas widget in tkinter to create a frame full of labels which has a scrollbar. All is working good except that the frame only expands to the size of the labels placed in it - I want the frame to expand to the size of the parent canvas.

This can easily be done if I use pack(expand = True) (which I have commented out in the code below) for the frame in the canvas but then then the scrollbar doesn't work.

Here's the appropriate bit of code:

    ...
    self.canvas = Canvas(frame, bg = 'pink')
    self.canvas.pack(side = RIGHT, fill = BOTH, expand = True)

    self.mailbox_frame = Frame(self.canvas, bg = 'purple')

    self.canvas.create_window((0,0),window=self.mailbox_frame, anchor = NW)

    #self.mailbox_frame.pack(side = LEFT, fill = BOTH, expand = True)

    mail_scroll = Scrollbar(self.canvas, orient = "vertical",
        command = self.canvas.yview)
    mail_scroll.pack(side = RIGHT, fill = Y)

    self.canvas.config(yscrollcommand = mail_scroll.set)

    self.mailbox_frame.bind("<Configure>", self.OnFrameConfigure)


def OnFrameConfigure(self, event):
    self.canvas.configure(scrollregion=self.canvas.bbox("all"))

I've also provided an image with colored frames so you can see what I'm getting at. The pink area is the canvas that needs filling by the mailbox_frame (You can see the scrollbar on the right):

Just for future reference in case anyone else needs to know:

        frame = Frame(self.bottom_frame)
        frame.pack(side = LEFT, fill = BOTH, expand = True, padx = 10, pady = 10)

        self.canvas = Canvas(frame, bg = 'pink')
        self.canvas.pack(side = RIGHT, fill = BOTH, expand = True)

        self.mailbox_frame = Frame(self.canvas, bg = 'purple')

        self.canvas_frame = self.canvas.create_window((0,0),
            window=self.mailbox_frame, anchor = NW)
        #self.mailbox_frame.pack(side = LEFT, fill = BOTH, expand = True)

        mail_scroll = Scrollbar(self.canvas, orient = "vertical", 
            command = self.canvas.yview)
        mail_scroll.pack(side = RIGHT, fill = Y)

        self.canvas.config(yscrollcommand = mail_scroll.set)

        self.mailbox_frame.bind("<Configure>", self.OnFrameConfigure)
        self.canvas.bind('<Configure>', self.FrameWidth)

    def FrameWidth(self, event):
        canvas_width = event.width
        self.canvas.itemconfig(self.canvas_frame, width = canvas_width)

    def OnFrameConfigure(self, event):
        self.canvas.configure(scrollregion=self.canvas.bbox("all"))

Set a binding on the canvas <Configure> event, which fires whenever the canvas changes size. From the event object you can get the canvas width and height, and use that to resize the frame.