Error: The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.0 Must issue a STARTTLS command first.
Description: This error occurs when you try to send an email using .Net code and the email account configured to send email is a GMAIL account which having https enabled. You can view the settings from tab Setting -> General -> Browser connection – Always use https.
Solution: Enable SSL to true from .Net code.
var client = new SmtpClient();
client.EnableSsl = true;
More discussion about secure connection in GMail and others: http://security.stackexchange.com/questions/6489/what-steps-do-gmail-yahoo-mail-and-hotmail-take-to-prevent-eavesdropping-on-e