Installed Version 2.9.0-beta-2222
But occured another Exception at same line
Invalid or unknown command response received. GenericNack, Status: ESME_ROK, Sequence

IList<SubmitMultiResp> resp = await _client.SubmitAsync(SMS.ForSubmitMulti()
.ServiceType("")
.Text("Multi Message")
.From ...

Single message sent successfully . For multi message below exception is occured.
Unable to cast object of type 'Inetlab.SMPP.PDU.GenericNack' to type 'Inetlab.SMPP.PDU.SubmitMultiResp'. at await _client.SubmitAsync

List<IAddress> destList = new List<IAddress>();
destList.Add(new SmeAddress ...