I checked out the events that are available to check for new mail (I never noticed them before because I use Outlook 98 at work which doesn't have these events - the events are only in Outlook 2000 onwards)
Anyway, the NewMail event is an Application event that only works for the Inbox but there is another event you can use. You want to use the ItemAdd event of the Items class.
The example below has been lifted straight out of the Microsoft Outlook VBA help file (if you search for ItemAdd you will get ItemAdded which is not the same - you have to navigate through the following items in the menu - Microsoft Outlook Visual Basic Reference --> Events --> I --> ItemAdd Event)
Anyway, here is Microsoft's sample code:
'============================
ItemAdd Event Example
In this example, when a new contact is added to the Contacts folder, the contact item is attached to a mail message and sent to a distribution list named Sales Team. The sample code must be placed in a class module, and the Initialize_handler routine must be called before the event procedure can be called by Microsoft Outlook.
Dim myOlApp As Outlook.Application
Public WithEvents myOlItems As Outlook.Items
Public Sub Initialize_handler()
Set myOlItems = myOlApp.GetNamespace("MAPI"

.GetDefaultFolder(olFolderContacts).Items
End Sub
Private Sub myOlItems_ItemAdd(ByVal Item As Object)
Dim myOlMItem As Outlook.MailItem
Dim myOlAtts As Outlook.Attachments
Set myOlMItem = myOlApp.CreateItem(olMailItem)
myOlMItem.Save
Set myOlAtts = myOlMItem.Attachments
' Add new contact to attachments in mail message
myOlAtts.Add Item, olByValue
myOlMItem.To = "Sales Team"
myOlMItem.Subject = "New contact"
myOlMItem.Send
End Sub
'================================
This should allow you to do what you need to do. In the Initialize_handler just set the items to the folder you want to check. I assume this will work - I haven't checked it.
Hope this helps.