NSight v184.108.40.206 has a known issue, which has been resolved in v220.127.116.11-033R and higher releases. If running v18.104.22.168 to v22.214.171.124, upgrade to v5.9.0 or higher if SMTP require security open. If upgrade is not feasible, one of the following security methods need to be implemented on NSight server SMTP settings as well as SMTP server:
ssl Use SMTP with SSL encryption
starttls Use SMTP with STARTTLS encryption
Username/password will be required on SMTP server and NSight SMTP configuration.