(56: response reading failed (errno: 0)) - this means that we did not get some response from the mail server that we expected. "we" is actually libcurl that is used in NetXMS to work with SMTP.
Since you use TLSMode=NONE which means that communication is unencrypted, you can install Wireshark and check what exactly communication is happening with the server. But it could be that server expects TLS, you can try other settings of TLSMode (and port)