C# 發郵件 服務器響應為: 5.7.0 Must issue a STARTTLS command first

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. d21sm2344514and.26

SMTP 服務器要求安全連接或客戶端未通過身份驗證。 服務器響應為: 5.7.0 Must issue a STARTTLS command first. i6sm4954143tid.5

今天用Gmail的發送郵件,返回這錯誤.查了下是因為Gmail要求把ssl加密打開.

加上這代碼:client.EnableSsl = true;