Not tested but should give you the general idea:

Private Sub Application_NewMail()

   Dim objOl As Outlook.Application
   Dim objNameSpace As Outlook.NameSpace

   Dim objMailitem As Outlook.MailItem
   Dim objFolder As MAPIFolder
   dim lPos as long

   Dim lIndex As Long

   Set objOl = New Outlook.Application
   Set objNameSpace = objOl.GetNamespace("MAPI")
   objNameSpace.Logon

   Dim strOrderBody As String

   On Error Resume Next
   Set objFolder = objNameSpace.GetDefaultFolder(olFolderInbox)
   lIndex = 1
   Do While lIndex <= objFolder.Items.Count
       Set objMailitem = objFolder.Items(lIndex)
       If Err.Number = 0 Then
          If objMailitem.UnRead  Then
               lpos = instr(1, objmailitem.body ,"yourphrase")
               do while lpos <> 0 and lpos < len(objmailitem.body)
                   if lpos > 0 then
                           objmailitem.body  = left (objmailitem.body
,lpos-1) & "****" & _
                           mid ( objmailitem.body , lpos )
                   end if
                   lpos = instr(lpos + len("yourphrase"), objmailitem.body
,"yourphrase")
               loop
                objmailitem.save
           end if

       end if
       lIndex = lIndex +1
   loop

end sub

Visit outlookcode.com - while vba is vba, because outlook uses so many
different windows and doesn't have a macro recorder for you to learn from,
it's not as simple as writing vba for excel (or word).