C# - Send Email using MX DNS Lookup - Smart Host

The following c# example codes demonstrate how to send email without specified SMTP server.

How does it work?

In general, we send email via specified SMTP server. How does the specified SMTP server know what address this email should be sent to? The answer is... it queries MX record of recipient’s domain via DNS lookup. It then forwards this email to the SMTP server queried from DNS server. If recipient’s server doesn’t work fine, sender’s SMTP server will send a failure-delivery report to the sender telling it failed to send out the email.

How does EASendMail SMTP component work with “Send email directly”? Firstly, it queries MX record for recipient address from DNS, then sends email to recipient’s email server directly. In short, if no SMTP server is specified in the code, EASendMail will send email to recipient directly. Since querying DNS server consumes CPU time and networking resource, the performance of “Send email directly” is lower than sending email with specified SMTP server. Moreover, nowadays more and more SMTP servers block email sent from dynamic IP address, so we don’t recommend you to use “Direct Send Email” except you have a static IP address or you encounter problem with your ISP SMTP server.

Every recipient may have different SMTP server, if there are multiple recipients in one message and you want to send email directly, you should send the email to the recipients one by one.

To implement this feature, you just need to put nothing to SMTP server address.

Installation

Before you can use the following codes, please download EASendMail SMTP Component and install it on your machine at first. Full sample proejcts are included in this installer.

Install from NuGet

You can also install the run-time assembly by NuGet. Run the following command in the NuGet Package Manager Console:

Install-Package EASendMail

Note

If you install it by NuGet, no sample projects are installed, only .NET assembly is installed.

Add Reference

To use EASendMail SMTP Component in your project, the first step is Add reference of EASendMail to your project. Please create or open your project with Visual Studio, then go to menu -> Project -> Add Reference -> .NET -> Browse..., and select EASendMail{version}.dll from your disk, click Open -> OK, the reference of EASendMail will be added to your project, and you can start to use it to send email in your project.

add reference in c#/vb.net/c++/cli/clr

.NET Assembly

Because EASendMail has separate builds for .Net Framework, please refer to the following table and choose the correct dll.

Separate builds of run-time assembly for .Net Framework 1.1, 2.0, 3.5, 4.0, 4.5 and .Net Compact Framework 2.0, 3.5.

File .NET Framework Version
EASendMail.dll Built with .NET Framework 1.1
It requires .NET Framework 1.1, 2.0, 3.5 or later version.
EASendMail20.dll Built with .NET Framework 2.0
It requires .NET Framework 2.0, 3.5 or later version.
EASendMail35.dll Built with .NET Framework 3.5
It requires .NET Framework 3.5 or later version.
EASendMaill40.dll Built with .NET Framework 4.0
It requires .NET Framework 4.0 or later version.
EASendMaill45.dll Built with .NET Framework 4.5
It requires .NET Framework 4.5 or later version.
EASendMailCF20.dll Built with .NET Compact Framework 2.0
It requires .NET Compact Framework 2.0, 3.5 or later version.
EASendMailCF35.dll Built with .NET Compact Framework 3.5
It requires .NET Compact Framework 3.5 or later version.

C# - Send Email using MX DNS Lookup - Smart Host - Example

The following example codes demonstrate sending email message using MX DNS lookup. In order to run it correctly, please change SMTP server, user, password, sender, recipient value to yours.

Note

To get full sample projects, please download and install EASendMail on your machine.

using System;
using System.Collections.Generic;
using System.Text;

// Add EASendMail namespace
using EASendMail;

namespace mysendemail
{
    class Program
    {
        static void Main(string[] args)
        {
            SmtpMail oMail = new SmtpMail("TryIt");
            SmtpClient oSmtp = new SmtpClient();

            // Set sender email address, please change it to yours
            oMail.From = "test@emailarchitect.net";

            // Set recipient email address, please change it to yours
            oMail.To = "support@emailarchitect.net";

            // Set email subject
            oMail.Subject = "test email from c# database queue";

            // Set email body
            oMail.TextBody = "this is a test email sent from c# project, do not reply";

            // Do not set SMTP server address
            SmtpServer oServer = new SmtpServer("");

            try
            {
                Console.WriteLine("start to send email to queue...");
                oSmtp.SendMail(oServer, oMail);
                Console.WriteLine("email was sent successfully!");
            }
            catch (Exception ep)
            {
                Console.WriteLine("failed to send email with the following error:");
                Console.WriteLine(ep.Message);
            }
        }
    }
}

With above code, if you get error like “5xx IP address rejected”, that means your IP address is blocked by the recipient’s SMTP server. You have to specify a SMTP server with user authentication to relay your email.

Important Notice

If your machine doesn’t have a static IP address, then I don’t suggest that you send email directly.

  • If your IP address is dynamic, most SMTP servers reject your connection due to anti-spam policy. We always suggest that your send email by a SMTP server that has a static internet IP address. When you relay email by your SMTP server, because you do user authentication at first before you send email to your SMTP server, so your SMTP server doesn’t reject your connection even your IP address is dynamic. Finally your SMTP server sends email to remote SMTP server. Because your SMTP server has a static IP, the email won’t be rejected by remote SMTP server.
Send email using DNS lookup in C#
  • If you encountered a temporal SMTP error (4xx), you should retry to send email later. That means you have to write the code to handle retry. So if you have a static IP address, I suggest that you use EASendMail Component + EASendMail Service, EASendMail service can send email directly or send email with specified SMTP server in background and handle delivery retry automatically.

To learn more detail about EASendMail Serivce, please have a look at Work with EASendMail Service (Email Queuing).

Appendix

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