Email Sender

• Download source code - 30.4 KB

Introduction

Usually, when we send an email, we need to login to a known email service provider's SMTP server and deliver the email using that server. If we add the send email functionality to a software, the user needs to configure an SMTP server address, the username, and the password. Why not send an email to the receiver's SMTP server directly? Because the receiver's server doesn't need authorization, otherwise you can only receive email when the sender knows your account password. I think many programmers have been asking this question, so I explored the issue.

The problem is we don't know which SMTP server is responsible for receiving emails for a given email address. The secret is that this information can be obtained from Domain Name System (DNS) servers. This seems simple; however, it needs a lot work to implement the DNS protocol (RFC 1035) because the .NET Framework doesn't support getting mail server info from DNS. Once we have the SMTP server address, we can send emails to the receiver directly!

Using the code

I partially implemented the DNS protocol in four classes: DnsMessage, DnsQuery, DnsResource, and DnsMessageCoder. Then, I wrapped the functionality in the DomainNameUtil class.

Now, EmailSender can use DomainNameUtil to get the mail server from the email address and send the email:

using System;
using System.Collections.Generic;
using System.Text;
using System.Net;
using System.Net.Mail;
namespace QiHe.CodeLib.Net
{
    public class EmailSender
    {
        /// <summary>
        /// Default SMTP Port.
        /// </summary>
        public static int SmtpPort = 25;

        public static bool Send(string from, string to, string subject, string body)
        {
            string domainName = GetDomainName(to);
            IPAddress[] servers = GetMailExchangeServer(domainName);
            foreach (IPAddress server in servers)
            {
                try
                {
                    SmtpClient client = new SmtpClient(server.ToString(), SmtpPort);
                    client.Send(from, to, subject, body);
                    return true;
                }
                catch
                {
                    continue;
                }
            }
            return false;
        }

        public static bool Send(MailMessage mailMessage)
        {
            string domainName = GetDomainName(mailMessage.To[0].Address);
            IPAddress[] servers = GetMailExchangeServer(domainName);
            foreach (IPAddress server in servers)
            {
                try
                {
                    SmtpClient client = new SmtpClient(server.ToString(), SmtpPort);
                    client.Send(mailMessage);
                    return true;
                }
                catch
                {
                    continue;
                }
            }
            return false;
        }

        public static string GetDomainName(string emailAddress)
        {
            int atIndex = emailAddress.IndexOf('@');
            if (atIndex == -1)
            {
                throw new ArgumentException("Not a valid email address", 
                                            "emailAddress");
            }
            if (emailAddress.IndexOf('<') > -1 && 
                emailAddress.IndexOf('>') > -1)
            {
                return emailAddress.Substring(atIndex + 1, 
                       emailAddress.IndexOf('>') - atIndex);
            }
            else
            {
                return emailAddress.Substring(atIndex + 1);
            }
        }

        public static IPAddress[] GetMailExchangeServer(string domainName)
        {
            IPHostEntry hostEntry = 
              DomainNameUtil.GetIPHostEntryForMailExchange(domainName);
            if (hostEntry.AddressList.Length > 0)
            {
                return hostEntry.AddressList;
            }
            else if (hostEntry.Aliases.Length > 0)
            {
                return System.Net.Dns.GetHostAddresses(hostEntry.Aliases[0]);
            }
            else
            {
                return null;
            }
        }
    }
}

The user interface is naive; multi-threading and message logging have not been done.

Points of interest

If you have a typo error in the sender email address or use a fake address, the email can still be sent, but this way, you can't expect the receiver to reply to you.

Some systems, e.g., SPF, will check if the sender's IP is declared as a domain name or not. This makes it unreliable to send the email to any address.

In addition, don't send spam emails using my tool or the underlying source code!

History

• 2008-06-05 - Article submitted.
• 2011-06-11 - Updated introduction text.