MailItem.Save sometimes seems doesnot works

Imports System.Runtime.InteropServices
Imports System.ComponentModel
Imports Outlook = Microsoft.Office.Interop.Outlook

'Add-in Express Add-in Module
<GuidAttribute("2DC698E6-9E5F-4CF7-8506-88CFB1BC5EAB"), ProgIdAttribute("MailItemSaveTest.AddinModule"), RunInstaller(True)> _
Public Class AddinModule
  Inherits AddinExpress.MSO.ADXAddinModule

#Region " Component Designer generated code. "
  'Required by designer
  Private components As System.ComponentModel.IContainer

  'Required by designer - do not modify
  'the following method
  Private Sub InitializeComponent()
    Me.components = New System.ComponentModel.Container
    '
    'AddinModule
    '
    Me.AddinName = "MailItemSaveTest"

    Me.SupportedApps = CType(( _
AddinExpress.MSO.ADXOfficeHostApp.ohaOutlook _
), AddinExpress.MSO.ADXOfficeHostApp)
  End Sub

#End Region

#Region " ADX automatic code "

  'Required by Add-in Express - do not modify
  'the methods within this region

  Public Overrides Function GetContainer() As System.ComponentModel.IContainer
    If components Is Nothing Then
      components = New System.ComponentModel.Container
    End If
    GetContainer = components
  End Function

  <ComRegisterFunctionAttribute()> _
  Public Shared Sub AddinRegister(ByVal t As Type)
    AddinExpress.MSO.ADXAddinModule.ADXRegister(t)
  End Sub

  <ComUnregisterFunctionAttribute()> _
  Public Shared Sub AddinUnregister(ByVal t As Type)
    AddinExpress.MSO.ADXAddinModule.ADXUnregister(t)
  End Sub

  Public Overrides Sub UninstallControls()
    MyBase.UninstallControls()
  End Sub

#End Region

  Public Sub New()
    MyBase.New()

    'This call is required by the Component Designer
    InitializeComponent()

    'Add any initialization after the InitializeComponent() call

  End Sub

  Public ReadOnly Property OutlookApp() As Outlook._Application
    Get
      Return CType(HostApplication, Outlook._Application)
    End Get
  End Property

  Private Sub AddinModule_AddinStartupComplete(ByVal sender As Object, ByVal e As System.EventArgs) Handles MyBase.AddinStartupComplete
    Dim itemsEvents As New OutlookItemsEventsClass1(Me)

    itemsEvents.ConnectTo(AddinExpress.MSO.ADXOlDefaultFolders.olFolderDrafts, True)
  End Sub
End Class

'Add-in Express Outlook Items Events Class
Public Class OutlookItemsEventsClass1
  Inherits AddinExpress.MSO.ADXOutlookItemsEvents

  Public Sub New(ByVal ADXModule As AddinExpress.MSO.ADXAddinModule)
    MyBase.New(ADXModule)
  End Sub

  Public Overrides Sub ProcessItemAdd(ByVal Item As Object)
    If Not TypeOf Item Is Outlook.MailItem Then Exit Sub

    Dim mail As Outlook.MailItem = CType(Item, Outlook.MailItem)
    Dim filename As String = IO.Path.GetTempFileName
    mail.SaveAs(filename)
    Dim entersoftCRMregID As String = EntersoftCRMserver.RegisterEmail(filename)
    mail.UserProperties.Add("EntersoftCRMregID", Outlook.OlUserPropertyType.olText, True).Value = entersoftCRMregID
    mail.FlagIcon = Outlook.OlFlagIcon.olPurpleFlagIcon
    mail.Save()
  End Sub

  Public Overrides Sub ProcessItemChange(ByVal Item As Object)
    'TODO: Add some code
  End Sub

  Public Overrides Sub ProcessItemRemove()
    'TODO: Add some code
  End Sub

End Class

Public Class EntersoftCRMserver
  Public Shared Function RegisterEmail(ByVal mailFilename As String) As String
    'process email
    '...
    IO.File.Delete(mailFilename)
    'return registration ID
    Return Guid.NewGuid.ToString
  End Function
End Class