C#使用System.Net.Mail类实现邮件发送

有些 SMTP 服务器要求在代表客户端发送电子邮件前验证客户端的身份。当此 SmtpClient 对象应该使用当前登录用户的默认凭据进行身份验证（如果服务器要求）时，请将此属性设置为 true。对于客户端应用程序，这是在大多数情况下都需要的行为。也可以使用应用程序和计算机配置文件指定凭据信息。有关更多信息，请参见 <mailSettings> 元素（网络设置）。

如果 UseDefaultCredentials 属性设置为 false，则连接到服务器时会将 Credentials 属性中设置的值用作凭据。如果 UseDefaultCredentials 属性设置为 false 并且尚未设置 Credentials 属性，则将邮件以匿名方式发送到服务器。UseDefaultCredentials 的默认值为false。

如果提供用于基本身份验证的凭据，则凭据将以明文形式发送到服务器。这可能会产生安全问题，因为凭据可以被他人看见从而加以利用。但是向163的邮箱发送邮件是必须要带上发送者的凭据信息，否则邮件是发送不出去的。

SmtpClient也可以这样建立:SmtpClient client=new SmtpClient(smtp_server)，因为邮件发送服务器的默认端口是25。

1、QQ邮箱向163邮箱发送邮件

private static void TimeEvent(object source, ElapsedEventArgs e)
{
   string smtp_server="smtp.qq.com";
   int port = 25;
   string mail_from = "***from_mail_username***@qq.com";
   string sender = "***from_mail_username***";
   string mail_to = "***to_mail_username***@163.com";
   string receiver = "***to_mail_username***@";
   string subject = "Title Test"
   string body = "Content Test";
   try
   {
   SendEmail(smtp_server, port, mail_from, sender, mail_to, receiver, subject, body);
   }
   catch(Exception ex)
   {
   MessageBox.Show(ex.Message);
   }
}

public static void SendEmail(string smtp_server, int port, string mail_from, string sender, string mail_to, string receiver, string subject, string body)
{
   MailAddress from = new MailAddress(mail_from, sender);
   MailAddress to = new MailAddress(mail_to, receiver);
   MailMessage message = new MailMessage(from, to);
   message.BodyEncoding = Encoding.UTF8;
   message.IsBodyHtml = true;
   message.Subject = subject;
   message.Body = body;

SmtpClient client = new SmtpClient(smtp_server, port);  
   client.Credentials = new NetworkCredential("***from_mail_username***@qq.com", "***pwd***");
   client.Send(message);
}

2、163邮箱向163邮箱发送邮件

private static void TimeEvent(object source, ElapsedEventArgs e)
{
   string smtp_server="smtp.163.com";
   int port = 25;
   string mail_from = "***from_mail_username***@163.com";
   string sender = "***from_mail_username***";
   string mail_to = "***to_mail_username***@163.com";
   string receiver = "***to_mail_username***";
   string subject = "Title Test"
   string body = "Content Test";
   try
   {
   SendEmail(smtp_server, port, mail_from, sender, mail_to, receiver, subject, body);
   }
   catch(Exception ex)
   {
   MessageBox.Show(ex.Message);
   }
}

public static void SendEmail(string smtp_server, int port, string mail_from, string sender, string mail_to, string receiver, string subject, string body)
{
   MailAddress from = new MailAddress(mail_from, sender);
   MailAddress to = new MailAddress(mail_to, receiver);
   MailMessage message = new MailMessage(from, to);
   message.BodyEncoding = Encoding.UTF8;
   message.IsBodyHtml = true;
   message.Subject = subject;
   message.Body = body;

SmtpClient client = new SmtpClient(smtp_server);  
   client.Credentials = new NetworkCredential("***from_mail_username***@163.com", "***pwd***");
   client.Send(message);
}

3、163邮箱向QQ邮箱发送邮件

private static void TimeEvent(object source, ElapsedEventArgs e)
{
   string smtp_server="smtp.163.com";
   int port = 25;
   string mail_from = "***from_mail_username***@163.com";
   string sender = "***from_mail_username***";
   string mail_to = "***to_mail_username***@qq.com";
   string receiver = "***to_mail_username***";
   string subject = "Title Test"
   string body = "Content Test";
   try
   {
   SendEmail(smtp_server, port, mail_from, sender, mail_to, receiver, subject, body);
   }
   catch(Exception ex)
   {
   MessageBox.Show(ex.Message);
   }
}

public static void SendEmail(string smtp_server, int port, string mail_from, string sender, string mail_to, string receiver, string subject, string body)
{
   MailAddress from = new MailAddress(mail_from, sender);
   MailAddress to = new MailAddress(mail_to, receiver);
   MailMessage message = new MailMessage(from, to);
   message.BodyEncoding = Encoding.UTF8;
   message.IsBodyHtml = true;
   message.Subject = subject;
   message.Body = body;

SmtpClient client = new SmtpClient(smtp_server);  
   client.Credentials = new NetworkCredential("***from_mail_username***@163.com", "***pwd***");
   client.Send(message);
}

4、QQ邮箱向QQ邮箱发送邮件

private static void TimeEvent(object source, ElapsedEventArgs e)
{
   string smtp_server="smtp.qq.com";
   int port = 25;
   string mail_from = "***from_mail_username***@qq.com";
   string sender = "***from_mail_username***";
   string mail_to = "***to_mail_username***@qq.com";
   string receiver = "***to_mail_username***";
   string subject = "Title Test"
   string body = "Content Test";
   try
   {
   SendEmail(smtp_server, port, mail_from, sender, mail_to, receiver, subject, body);
   }
   catch(Exception ex)
   {
   MessageBox.Show(ex.Message);
   }
}

public static void SendEmail(string smtp_server, int port, string mail_from, string sender, string mail_to, string receiver, string subject, string body)
{
   MailAddress from = new MailAddress(mail_from, sender);
   MailAddress to = new MailAddress(mail_to, receiver);
   MailMessage message = new MailMessage(from, to);
   message.BodyEncoding = Encoding.UTF8;
   message.IsBodyHtml = true;
   message.Subject = subject;
   message.Body = body;

SmtpClient client = new SmtpClient(smtp_server);  
   client.Credentials = new NetworkCredential("***from_mail_username***@qq.com", "***pwd***");
   client.Send(message);
}

来源：https://www.cnblogs.com/lhb25/archive/2008/10/30/1323042.html