The SmtpClient class supports both the "smtp" and "smtps" protocols. The "smtp" protocol makes a clear-text connection to the SMTP server and does not use SSL or TLS unless the SMTP server supports the STARTTLS extension. The "smtps" protocol, however, connects to the SMTP server using an SSL-wrapped connection.
The connection established by any of the Connect methods may be re-used if an application wishes to send multiple messages to the same SMTP server. Since connecting and authenticating can be expensive operations, re-using a connection can significantly improve performance when sending a large number of messages to the same SMTP server over a short period of time.