Private Sub Кнопка0_Click()
    Dim olk As Outlook.Application
    
    On Error Resume Next
    Set olk = GetObject(, "Outlook.Application")
    If Err.Number = 0 Then
        On Error GoTo Err_
        If Not olk.ActiveExplorer Is Nothing Then
            If Not olk.ActiveExplorer.ActiveInlineResponse Is Nothing Then
                If TypeName(olk.ActiveExplorer.ActiveInlineResponse) = "MailItem" Then
                    If Not olk.ActiveExplorer.ActiveInlineResponse.Sent Then ОбработкаПисьма olk.ActiveExplorer.ActiveInlineResponse
                End If
            End If
        End If
        If Not olk.ActiveInspector Is Nothing Then
            If Not olk.ActiveInspector.CurrentItem Is Nothing Then
                If TypeName(olk.ActiveInspector.CurrentItem) = "MailItem" Then
                    If Not olk.ActiveInspector.CurrentItem.Sent Then ОбработкаПисьма olk.ActiveInspector.CurrentItem
                End If
            End If
        End If
    Else
        MsgBox "Вначале создайте новое письмо в Outlook!", vbInformation, "Внимание"
    End If
    Exit Sub

Err_:
    MsgBox Err.Description, vbCritical, "Ошибка"
End Sub

Private Sub ОбработкаПисьма(mi As Outlook.MailItem)
    mi.Subject = "RE: My Subject " & Now
    mi.Body = "Тра-та-та"
End Sub

Private Sub Кнопка0_Click()
    Dim olk As Outlook.Application, oi As Object
    
    On Error Resume Next
    Set olk = GetObject(, "Outlook.Application")
    If Err.Number = 0 Then
        On Error GoTo Err_
        If olk.ActiveExplorer Is Nothing Then
            If Not olk.ActiveInspector Is Nothing Then Set oi = olk.ActiveInspector.CurrentItem
        Else
            Set oi = olk.ActiveExplorer.ActiveInlineResponse
        End If
    
        If Not oi Is Nothing Then
            If TypeName(oi) = "MailItem" Then
                If Not oi.Sent Then
                    ОбработкаПисьма oi
                    Exit Sub
                End If
            End If
        End If
    End If
    MsgBox "Вначале создайте новое письмо в Outlook!", vbInformation, "Внимание"
    Exit Sub

Err_:
    MsgBox Err.Description, vbCritical, "Ошибка"
End Sub

Private Sub ОбработкаПисьма(mi As Outlook.MailItem)
    mi.Subject = "RE: My Subject " & Now
    mi.Body = "Тра-та-та"
End Sub