Option Explicit
Private WithEvents Items As Outlook.Items

Private Sub Application_Startup()
Dim Ns As Outlook.NameSpace
Set Ns = Application.GetNamespace("MAPI")
Set Items = Ns.GetDefaultFolder(olFolderInbox).Items
End Sub

Private Sub Items_ItemAdd(ByVal Item As Object)
If InStr(1, Item.Subject, "Accepted: ") Then
GoTo SendReply
Else
Exit Sub
End If

SendReply:
Dim oRespond As Outlook.MailItem

Dim RmSize As String
Dim strSubject As String
Dim arrRoomName As Variant
Dim arrRmSize As Variant
Dim i As Long
Dim meetinglocation As String
Dim meetingstart
Dim propertyAccessor As Outlook.propertyAccessor
'  http://schemas.microsoft.com/mapi/id/{00062002-0000-0000-C000-000000000046}/8208001E
' start http://schemas.microsoft.com/mapi/proptag/0x00600040

Set propertyAccessor = Item.propertyAccessor
 meetinglocation = propertyAccessor.GetProperty("http://schemas.microsoft.com/mapi/id/{00062002-0000-0000-C000-000000000046}/8208001E")
meetingstart = propertyAccessor.GetProperty("http://schemas.microsoft.com/mapi/proptag/0x00600040")
Debug.Print meetinglocation
' Set up the array using room names and sizes
'
arrRoomName = Array("Room 1", "Alpha", "Board Room", "Conference Room", "Delta", "Meeting Room - North")
arrRmSize = Array("2", "3", "6", "4", "50", "35")

' Go through the array and look for a match, then do something
For i = LBound(arrRoomName) To UBound(arrRoomName)
If InStr(meetinglocation, arrRoomName(i)) Then
RmSize = arrRmSize(i)
strSubject = Format(meetingstart, "yyyymmddhhnn ") & Item.Subject

' Start Registry method
' Replace with text code if desired
' HKEY_CURRENT_USER\Software\VB and VBA Program Settings\Outlook\Meeting Counts
Dim sAppName As String
Dim sSection As String
Dim sKey As String
Dim lRegValue As Long
Dim iDefault As Integer
sAppName = "Outlook"
sSection = "Meeting Counts"
sKey = strSubject

' The default starting number.
iDefault = 0 ' adjust as needed

' Get stored registry value, if any.
lRegValue = GetSetting(sAppName, sSection, sKey, iDefault)
Debug.Print sAppName, sSection, sKey, iDefault
' If the result is 0, set to default value.
If lRegValue = 0 Then lRegValue = iDefault

' Increment and update number.
SaveSetting sAppName, sSection, sKey, lRegValue + 1

If lRegValue > RmSize Then

' End Registry code

Set oRespond = Application.CreateItem(olMailItem)
With oRespond
.Recipients.Add Item.SenderEmailAddress
.Subject = "Sorry: " & strSubject
.Body = " Sorry, the room is full. You're on the waiting list. " & "You are " & lRegValue - RmSize & " on the waiting list."
' use .display for testing, .send after it is working as desired
'.Display
.Send
End With

Set oRespond = Nothing
End If
End If
Next i
End Sub