Unexpected End of Stream

Em client worked without fault until Sunday but now reports an Authentication failure due to "Unexpected end of stream.  Log and screen shot below:

06:54:57 xxx@tiscali.co.uk [SMTP]  Connecting: To xxx@tiscali.co.uk

06:54:58 xxx@tiscali.co.uk [SMTP]  MailExceptions.LoginException: The authentication failed due to the following reason:
06:54:58     "Unexpected end of stream.
06:54:58    at MailClient.Smtp.SmtpSendCommand.Authenticate(WorkerStatus status)
06:54:58    at MailClient.Smtp.SmtpSendCommand.Connect(WorkerStatus status)
06:54:58    at MailClient.Smtp.SmtpSendCommand.Execute(WorkerStatus status)
06:54:58    at MailClient.Commands.Command.Process(WorkerStatus status)