Recently one of our developers was facing this error when trying to connect with a secure SMTP server to send emails via C# code:
"Email Sending Failed. Reason: Failure sending mail. Inner: System.IO.IOException: Unable to read data from the transport connection: net_io_connectionclosed."
We had setup the credentials and the email settings properly, and confirmed them multiple times. The email port was set to 465 for secure SMTP. We just could not find the real reason, but when trying to fix this error we changed the port to 587 from 465, it worked! 465 port is used for SSL, and 587 for TLS, and our email server was using TLS. So for us, simply changing the port to 587 worked.