Private Sub Application_NewMailEx(ByVal EntryIDCollection As String)
    Dim objNamespace As Outlook.NameSpace
    Dim objItem As Object
    Dim objMail As Outlook.MailItem
    Dim EntryID As String
    Dim arrEntryIDs() As String
    Dim i As Integer

    ' EntryIDCollectionを分割
    arrEntryIDs = Split(EntryIDCollection, ",")

    Set objNamespace = Application.GetNamespace("MAPI")
    
    ' 各メールに対して処理
    For i = LBound(arrEntryIDs) To UBound(arrEntryIDs)
        EntryID = arrEntryIDs(i)
        Set objItem = objNamespace.GetItemFromID(EntryID)
        
        ' メールアイテムの場合
        If TypeOf objItem Is Outlook.MailItem Then
            Set objMail = objItem
            
            ' 指定の件名をチェック
            If objMail.Subject = "指定した件名" Then
                Dim replyMail As Outlook.MailItem
                Dim bodyText As String
                
                ' 本文の文字列置換
                bodyText = objMail.Body
                bodyText = Replace(bodyText, "aaa", "xxx")
                
                ' 返信メール作成
                Set replyMail = objMail.Reply
                replyMail.Body = bodyText
                replyMail.To = "返信先のメールアドレス@example.com" ' 指定アドレス
                replyMail.Send
            End If
        End If
    Next i

    ' 解放
    Set objMail = Nothing
    Set objItem = Nothing
    Set objNamespace = Nothing
End Sub