It could be a temporary server issue, which will resolve itself after a while.
Or something may be interfering with the connection to the server. As a test, can you completely disable any anti-virus, firewall, proxy or VPN, then try send again. Sometimes even restarting your router may resolve this.
You can also try other SMTP ports. Here are the other two port/security policy options. Make a change then click on Save & Close and try send a message:
Port 587 = Force usage of SSL/TLS
Port 465 = Use SSL/TLS on special port (legacy)