email - Using ReceivedTime in Outlook VBA to count yesterday's mail

I want to flash up a message box with the amount of mail received yesterday.

The code I have at the moment is:

Public Sub YesterdayCount()

Set ns = Application.GetNamespace("MAPI")
Set outApp = CreateObject("Outlook.Application")

Set outNS = outApp.GetNamespace("MAPI")

Dim Items As Outlook.Items
Dim MailItem As Object

Dim yestcount As Integer
yestcount = 0

Set Folder = outNS.Folders("Correspondence Debt Advisor Queries").Folders("Inbox")
Set Items = Folder.Items


For Each Item In Items
    If MailItem.ReceivedTime = (Date - 1) Then
        yestcount = yestcount + 1
    End If
Next

MsgBox yestcount

End Sub

The problem is with the line:

If MailItem.ReceivedTime = (Date - 1) Then

The error says that an object variable is not set, but I can't fathom this after researching.