System.Net.Mail.SmtpClient 在发送电子邮件时使用什么类型的身份验证？

Question

Hopefully this question makes sense. I have some C# code that sends an email message given the username, password, and domain. How do I know what type of authentication it's using?

When I looked at the MSDN page for SmtpClient.Credentials Property it said that if you use basic authentication then credentials are sent over as plain text. Is the below code using basic authentication? MSDN page: https://msdn.microsoft.com/en-us/library/system.net.mail.smtpclient.credentials(v=vs.110).aspx

SmtpClient smtpClient = new SmtpClient();
NetworkCredential cred = new NetworkCredential("username", "password"); 
MailMessage message = new MailMessage(); 
MailAddress fromEmail = new MailAddress("from@yourdomain.com"); 

smtpClient.Host = "mail.mydomain.com";
smtpClient.UseDefaultCredentials = false;
smtpClient.Credentials = cred;

message.From = fromEmail;
message.Subject = "my subject";

message.IsBodyHtml = true;
message.Body = "hello world!";
message.To.Add("toEmail@anything.com"); 

smtpClient.Send(message);

Answer 1

Typically SmtpClient will alway pick the "best" available authentication method (the SMTP server will tell the in the answer to the EHLO command). So we can't really answer without knowing what the server actually supports.

SmtpClient can choose from following methods: Negotiate, NTLM, Digest and Login

Here's some interesting reading about how to force SmtpClient to use a specific authentication method (slightly hack-ish btw.): http://blogs.msdn.com/b/knom/archive/2008/04/16/hacking-system-net-mail-smtpclient.aspx

Answer 2

This method will use basic authentification and will send the code via unsecure channel. This is because:

The SmtpClient class only supports the SMTP Service Extension for Secure SMTP over Transport Layer Security as defined in RFC 3207. In this mode, the SMTP session begins on an unencrypted channel , then a STARTTLS command is issued by the client to the server to switch to secure communication using SSL. See RFC 3207 published by the Internet Engineering Task Force (IETF) for more information.

You can use the SSL for such situations:

The EnableSsl property specifies whether SSL is used to access the specified SMTP mail server.

Also please note that

You can use ClientCertificates to specify which client certificates should be used to establish the SSL connection. The ServerCertificateValidationCallback allows you to reject the certificate provided by the SMTP server. The SecurityProtocol property allows you to specify the version of the SSL protocol to use.

Answer 3

This is an old thread, but I don't see my problem addressed here. I have a Windows Server using the soon-to-be-deprecated System.Net.Mail.SmtpClient. Because my emails are going through a load balancer which does not support AUTH LOGIN (multi-line authentication), I set my postfix servers to use AUTH PLAIN (single-line). Can I get SmtpClient to send (after it successfully sets up STARTTLS) to send AUTH PLAIN??? It does not seem to be doing so. Can it???

Maybe my question should be in a new thread, but I see comments in this thread about the SMTP server sending back AUTH options for the client to pick the best. And my SMTP server is sending back AUTH PLAIN in the list (as best/only), but the client does not seem to be picking it.