Unable to send message : Reason : org.columbia.ristretto.auth.authinticationException : org.columbia.ristretto.smtpException:5.51 Error:Already Authenticated

Unable to send message : Reason : org.columbia.ristretto.auth.authinticationException : org.columbia.ristretto.smtpException:5.51 Error:Already Authenticated

Hi, 

Since morning we are facing some issue with Mail. We are unable to sent email. Error Massage coming 
Unable to send message : Reason : org.columbia.ristretto.auth.authinticationException : org.columbia.ristretto.smtpException:5.51 Error:Already Authenticated