fix #15

In MainForm.cs 2396
```cs
- menuNotReadYet.Enabled = !(checkNotYetReadMail && listView1.SelectedItems.Count == 0 && listView1.Columns[0].Text == "名前");
+ menuNotReadYet.Enabled = !checkNotYetReadMail && (listView1.SelectedItems.Count > 0 && listView1.Columns[0].Text != "名前");

```