It is safer to run such a macro manually. The following will delete all messages in a folder selected from the macro that are older than the indicated number of days and are read.
Sub DeleteOldMessages()
Dim olFolder As Folder
Dim olDate As Date
Dim olItems As Outlook.Items
Dim strDate As String
Dim i As Long
Dim iDays As Integer
iDays = 3
olDate = Format(DateAdd("d", -iDays, Now()), "Short Date")
Set olFolder = Application.Session.PickFolder
strDate = "[Received] <= """ & olDate & """"
Set olItems = olFolder.Items.Restrict(strDate)
For i = olItems.Count To 1 Step -1
If TypeName(olItems(i)) = "MailItem" And _
olItems(i).UnRead = False Then
olItems.Item(i).Delete
End If
Next
Set olItems = Nothing
Set olFolder = Nothing
lbl_Exit:
Exit Sub
End Sub