var message = new MimeMessage();
message.From.Add(new MailboxAddress("xxx@163.com"));
message.To.Add(new MailboxAddress("xxx@163.com"));
message.Subject = "星期天去哪里玩？";
message.Body = new TextPart("plain") { Text = "我想去故宫玩，如何" };
using (var client = new SmtpClient())
// For demo-purposes, accept all SSL certificates (in case the server supports STARTTLS)
client.ServerCertificateValidationCallback = (s, c, h, e) => true;
client.Connect("smtp.163.com", 587, true);
// Note: since we don't have an OAuth2 token, disable
// the XOAUTH2 authentication mechanism.
client.AuthenticationMechanisms.Remove("XOAUTH2");
// Note: only needed if the SMTP server requires authentication
client.Authenticate("xxx@163.com", "password");
client.Send(message);
client.Disconnect(true);
catch(Exception ex)
这里需要注意几点：
1. 需要用587端口，并且开启SSL
2. 检查邮箱设置里面是否开启 POP3/SMTP、IMAP/SMTP服务
参考链接：
https://stackoverflow.com/questions/44305186/the-smtp-server-has-unexpectedly-disconnected-in-mailkit
http://www.cnblogs.com/savorboard/p/aspnetcore-email.html
