Smtp4dev - the fake SMTP email server for development and testing

Sending email is one of popular features in a business application. Email's content and format have to test carefully before send out to customer. In the past, I often tested sending email by using Gmail's smtp, but this way would require sending some information out. In this guide, we'll explore SMTP4dev, an open-source application that simplifies email testing for developers.

Smtp4dev Instruction

smtp4dev is an open-source application that simulates an SMTP email server to test and debug emails, it lets you test your SMTP mail server. smtp4dev can run on Windows, Linux, Mac OS-X (any where .NET Core is available). you can test send mail without any real SMTP mail server.

• github: https://github.com/rnwood/smtp4dev
• BSD-3-Clause license

Smtp4dev Installation

If you don't want to use the dotnet global tool or Docker (see below if you know what those mean), you can download a standalone (or .NET Core runtime dependent) release from github.

https://github.com/rnwood/smtp4dev/releases

I will use "Windows x64 binary standalone - Desktop app edition" in this guide.

after download "Rnwood.smtp4dev.desktop-win-x64-3.7.0-ci20250120117.zip", unzip it, and run Rnwood.Smtp4dev.Desktop.exe

Smtp4dev Configuration

A program that you want to send testing mail to smtp4dev, you need to configure the host/address and port number where smtp4dev is running.

click on Icon on Right-Top, and setting as below:

Testing send mail from my application

I write a simple code to send email by c#

using System.Net;
using System.Net.Mail;

public class Program
{
  public static void Main(string[] args)
  {
     // Replace with your actual email settings
     string senderEmail = "your_email@example.com";
     string senderPassword = "your_email_password";
     string recipientEmail = "recipient_email@example.com";
     string subject = "Test Email";
     string body = "This is a test email sent from C#.";
     string smtpHost = "localhost"; 
     int smtpPort = 25; 
     bool enableSsl = false; // or true

     try
     {
          using (var smtpClient = new SmtpClient(smtpHost, smtpPort))
          {
               smtpClient.EnableSsl = enableSsl;
               smtpClient.Credentials = new NetworkCredential(senderEmail, senderPassword);

               using (var mailMessage = new MailMessage(senderEmail, recipientEmail, subject, body))
               {
                    smtpClient.Send(mailMessage);
                    Console.WriteLine("Email sent successfully!");
               }
          }
     }
     catch (SmtpException ex)
     {
          Console.WriteLine($"SMTP Error: {ex.Message}");
     }
     catch (Exception ex)
     {
     Console.WriteLine($"Error sending email: {ex.Message}");
     }

     Console.ReadKey(); // Keep the console window open
  }
}

when I run above code, I can see the sending email in Smtp4dev

Smtp4dev features

• OpenAPI/Swagger API
• IMAP access to retrieve and delete messages
• SMTP session logging
• UTF8 support
• Viewport size switcher to simulate mobile etc
• Multipart MIME inspector
• HTML compatibility report and HTML validation
• TLS/SSL with implicit and STARTTLS modes and auto self-signed cert generation
• Authentication
• Multiple mailboxes along with rules to control what message goes where
• Reply, compose and relay messages including rules to auto relay
• Scripting expressions including error simulation

refer: https://github.com/rnwood/smtp4dev