SmtpClient也可以直接将电子邮件发送给收件人吗？

Question

SmptClient can be used to send an email via relay server . But is SmtpClient also able to send email directly to the receiver and not via relay server ?

EDIT

Any ideas how SmtpCLient needs to be configured to be able to send emails directly to the receiver ?

I tried with the following code but I got " The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. "

public class EmailService : IIdentityMessageService
{
    public Task SendAsync(IdentityMessage message)
    {
     // var credentialUserName = "myAccount@gmail.com";
        var sentFrom = "myAccount@gmail.com";
     // var pwd = "myPwd";

        System.Net.Mail.SmtpClient client = 
            new System.Net.Mail.SmtpClient("smtp.gmail.com");

        client.Port = 587;
        client.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network;
        client.UseDefaultCredentials = false;

     /* System.Net.NetworkCredential credentials = 
            new System.Net.NetworkCredential(credentialUserName, pwd);
     */
        client.EnableSsl = true;
     // client.Credentials = credentials;

        var mail = 
            new System.Net.Mail.MailMessage(sentFrom, message.Destination);

        mail.Subject = message.Subject;
        mail.Body = message.Body;

        return client.SendMailAsync(mail);
    }
}

SECOND EDIT:

Thanx, it works now. App sent email directly ( and not via myAccount@gmail.com ) to otherAccount@gmail.com . Here's the code:

public class EmailService : IIdentityMessageService
{
    public Task SendAsync(IdentityMessage message)
    {
        var sentFrom = "myAccount@gmail.com";

        System.Net.Mail.SmtpClient client = 
            new System.Net.Mail.SmtpClient("gmail-smtp-in.l.google.com");

        client.Port = 25;
        client.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network;
        client.UseDefaultCredentials = false;

        client.EnableSsl = true;


        var mail = 
            new System.Net.Mail.MailMessage(sentFrom, message.Destination);

        mail.Subject = message.Subject;
        mail.Body = message.Body;

        return client.SendMailAsync(mail);
    }
}

Thank you

Answer 1

Short answer: yes!

A relay server is just a server that is configured to accept all your emails and pass them on to the right destination. You can equally well contact the right destination server directly and delivery the email there.

This, of course, provided there is no firewall issues preventing you from contacting the destination server directly.

EDIT

The server smtp.gmail.com is for gmail users to send (outgoing) emails, ie you must authenticate with your gmail username and password in order to be allowed to send an email that way, but if you do that you can send to any recipient, ie also non-gmail addresses.

I understood your original question to mean you would like to send emails to (in this case) a gmail-address without using a proxy. In that case your client should behave as any arbitrary email server that is trying to send to a gmail-address, ie it should connect to one of official incoming SMTP servers for the domain as given by MX-records in the DNS. Eg one of gmail's MX-records points to gmail-smtp-in.l.google.com , and if you connect to port 25 of that server you can submit an email to a gmail-address (and you can also completely spoof the sending address, but then spam-filtering might cause your email to not be delivered).

My caveat about firewall issues is to interpreted as this: most ISPs disallow outgoing TCP connections to port 25 to other hosts than their own servers. This is just because of the above mentioned spoofing possibility, ie if your ISP allows you to make TCP connections to port 25 of other email servers, you can use that to send spam. Therefore your ISP might not allow you to do that, and instead you should relay your emails via your ISP so they can take appropriate measures if you try to spam people.