Please modify your code to log into the logwriter like this and send us the created log as well as content of the ex.ToString():
    try
    {
        Imap clientt = new Imap();
        clientt.LogWriter = new Rebex.FileLogWriter(@"c:\temp\log.txt", Rebex.LogLevel.Debug);
        clientt.Connect("imap.yandex.ru");
        clientt.Login("elseeker@yandex.ru", "******");
        clientt.SelectFolder("Inbox");
        ImapMessageCollection fromRebex = clientt.Search
        (
            ImapSearchParameter.From("embarcadero.com")
        );
        MessageBox.Show("2");
        MessageBox.Show("messages " + fromRebex.Count);
    }
    catch (Exception ex){ MessageBox.Show(ex.ToString()); }
Seeing the log would enable us to see what is going on and help you solve this issue.