Solarwinds Real-Time Configuration Change Detection Email Notification

I have install Solarwinds include NCM module. I would like to config the Real-Time Configuration Change Detection feature. I have configured all parameter following the configuration wizard. I tested to change some configuration one my Switches but I didn't get any notified email from Solarwinds. I found some error in NCM BL Logs. Please kindly see the error as below:

2018-07-31 15:00:21,270 [37] ERROR SolarWinds.Cirrus.BusinessLayer.RTNHelper - ExecuteRtn errorSystem.Net.Mail.SmtpFailedRecipientException: Mailbox unavailable. The server response was: 5.7.1 Unable to relay

I think this is an issue of Email Server. But I don't how to fix it. Does anybody meet this issue before?

Thanks,

Putmano

2018-07-31 15:00:21,270 [37] ERROR SolarWinds.Cirrus.BusinessLayer.RTNHelper - ExecuteRtn errorSystem.Net.Mail.SmtpFailedRecipientException: Mailbox unavailable. The server response was: 5.7.1 Unable to relay