Dim newToolBar As Office.CommandBar
Dim firstButton As Office.CommandBarButton
Dim secondButton As Office.CommandBarButton
Dim selectExplorers As Outlook.Explorers

Private Sub ThisAddIn_Startup(ByVal sender As Object, _
        ByVal e As System.EventArgs) Handles Me.Startup
    selectExplorers = Me.Application.Explorers()
    AddHandler selectExplorers.NewExplorer, AddressOf _
        Me.NewExplorer_Event
    AddToolbar()
End Sub

Private Sub NewExplorer_Event(ByVal new_Explorer _
    As Outlook.Explorer)
    new_Explorer.Activate()
    newToolBar = Nothing
    Call Me.AddToolbar()
End Sub

Private Sub AddToolbar()

    Dim button_1 As Office.CommandBarButton
    Dim button_2 As Office.CommandBarButton
    If newToolBar Is Nothing Then
        Dim cmdBars As Office.CommandBars = _
            Me.Application.ActiveExplorer().CommandBars
        newToolBar = cmdBars.Add("NewToolBar", _
            Office.MsoBarPosition.msoBarTop, False, True)
    End If
    Try
        button_1 = CType(newToolBar.Controls.Add(1),  _
            Office.CommandBarButton)
        With button_1
            .Style = Office.MsoButtonStyle.msoButtonCaption
            .Caption = "Button 1"
            .Tag = "Button1"
        End With
        If Me.firstButton Is Nothing Then
            Me.firstButton = button_1
            AddHandler firstButton.Click, AddressOf ButtonClick
        End If
        button_2 = CType(newToolBar.Controls.Add(1),  _
            Office.CommandBarButton)
        With button_2
            .Style = Office.MsoButtonStyle.msoButtonCaption
            .Caption = "Button 2"
            .Tag = "Button2"
        End With
        If Me.secondButton Is Nothing Then
            Me.secondButton = button_2
            AddHandler secondButton.Click, AddressOf ButtonClick
        End If
        newToolBar.Visible = True
    Catch ex As Exception
        MessageBox.Show(ex.Message)
    End Try
End Sub

Private Sub ButtonClick(ByVal ctrl As Office.CommandBarButton, _
        ByRef Cancel As Boolean)
    MessageBox.Show("You clicked: " + ctrl.Caption)
End Sub