Сервер отклонил адрес отправителя

Question

Я смог отправлять электронные письма с помощью локального smtp, но при попытке отправить с помощью gmail он не работает.

ОШИБКА:

"The server rejected the sender address. The server response was: 530 5.7.0 
Must issue a STARTTLS command first. pj7sm14546972pbb.96 - gsmtp\r\n" 

C#:

public static void SendEmail() 
{ 
    MailMessage mailMsg = new MailMessage(); 
    SmtpClient smtpClient = new SmtpClient(); 
    mailMsg.From = "donotreply@admin.com"; //also tried smtpusername here 
    mailMsg.To = strToAddress; 
    mailMsg.Subject = strSubject; 
    mailMsg.Body = strBody; 
    smtpClient.Port = 587; 
    smtpClient.EnableSsl = true; 
    smtpClient.Credentials = new NetworkCredential(smtpusername, smtppassword); 
    //smtpusername & smtppassword are valid gmail credentials 
    SmtpMail.SmtpServer = "smtp.gmail.com"; 
    SmtpMail.Send(mailMsg); 
} 

Answer 1

If the EnableSsl property is set to true, and the SMTP mail server does not advertise STARTTLS in the response to the EHLO command, then a call to the Send or SendAsync methods will throw an SmtpException.

https://msdn.microsoft.com/en-us/library/system.net.mail.smtpclient.enablessl(v=vs.110).aspx