An error occurred. object reference not set to an instance of object .

I cant send e-mail after update ver 7.1.29274.0.

An error occurred “object reference not set to an instance of object”.

Check this log text.

오전 8:11:04 o???@???.com [POP3]  Connecting: To o???@???.com
오전 8:11:04 o???@???.com [SMTP]  Connecting: To o???@???.com
오전 8:11:04   System.NullReferenceException: Object reference not set to an instance of an object.
오전 8:11:04    at MailClient.Authentication.Mechanisms.Gssapi.GssapiSession.AcquireCredentials(String packageName, String user, String domain, String password)
오전 8:11:04    at MailClient.Authentication.Mechanisms.GssapiMechanism.Client…ctor(ClientCredential credential, String serviceType, String serverName, String protocolName)
오전 8:11:04    at MailClient.Authentication.Mechanisms.GssapiMechanism.CreateClient(ClientCredential credential, String serviceType, String serverName, TransportContext context)
오전 8:11:04    at MailClient.Protocols.Smtp.SmtpSendCommand.Authenticate(WorkerStatus status)
오전 8:11:04    at MailClient.Protocols.Smtp.SmtpSendCommand.Connect(WorkerStatus status)
오전 8:11:04    at MailClient.Protocols.Smtp.SmtpSendCommand.Execute(WorkerStatus status)
오전 8:11:04    at MailClient.Commands.Command.Process(WorkerStatus status)
오전 8:11:04 o???@???.com [POP3]  Connected: To o???@???.com
오전 8:11:04 o???@???.com [POP3]  Downloading messages: From o???@???.com
오전 8:11:04 o???@???.com [POP3]  Disconnected: From o???@???.com
오전 8:11:04 o???@???.com [POP3]  Disconnected: Done