Email Management with .NET 9 and C# using MailKit

📌 Introduction

Managing emails programmatically is a common requirement in many applications. In .NET 9, the SmtpClient class is obsolete and no longer recommended. Instead, MailKit and MimeKit provide a modern, efficient, and flexible way to handle email communication.

This guide will help you configure and use MailKit to send emails securely and efficiently.

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✅ Prerequisites

Before implementing email-sending functionality, ensure you have:

✔ A .NET 9 application (Console, Web, or Windows Forms)
✔ An SMTP server (e.g., Gmail, Outlook, or your own SMTP service)
✔ Valid SMTP credentials (username and password) or an OAuth2 token
✔ Secure storage for credentials (environment variables, Azure Key Vault, AWS Secrets Manager)

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🔧 Setting Up MailKit in C

1️⃣ Install Required Package

Run the following command to install MailKit:

 dotnet add package MailKit

2️⃣ Securely Store SMTP Credentials

❌ Bad practice: Hardcoding credentials in your code.
✅ Good practice: Use environment variables:

Set Environment Variables (Windows/Linux/macOS)

export SMTP_HOST=smtp.example.com
export SMTP_PORT=587
export SMTP_USER=[e-mail]
export SMTP_PASSWORD=your-password

Read Environment Variables in C#

var smtpHost = Environment.GetEnvironmentVariable("SMTP_HOST");
var smtpPort = int.Parse(Environment.GetEnvironmentVariable("SMTP_PORT"));
var smtpUser = Environment.GetEnvironmentVariable("SMTP_USER");
var smtpPassword = Environment.GetEnvironmentVariable("SMTP_PASSWORD");

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📤 Sending an Email

using MailKit.Net.Smtp;
using MailKit.Security;
using MimeKit;
using System;

class Program
{
    static void Main()
    {
        try
        {
            var message = new MimeMessage();
            message.From.Add(new MailboxAddress("Your Name", smtpUser));
            message.To.Add(new MailboxAddress("Recipient", "[e-mail]"));
            message.Subject = "Test Email";
            message.Body = new TextPart("plain") { Text = "This is a test email sent from a .NET 9 application." };

            using var client = new SmtpClient();
            client.Connect(smtpHost, smtpPort, SecureSocketOptions.StartTls);
            client.Authenticate(smtpUser, smtpPassword);
            client.Send(message);
            client.Disconnect(true);

            Console.WriteLine("Email sent successfully!");
        }
        catch (Exception ex)
        {
            Console.WriteLine($"Error: {ex.Message}");
        }
    }
}

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🔒 SecureSocketOptions Overview

Option	Description
None	No encryption.
SslOnConnect	Uses SSL/TLS immediately upon connection.
StartTls	Starts unencrypted, then upgrades to TLS (Recommended).
Auto	Automatically selects the best option.

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📎 Sending Emails with Attachments

var message = new MimeMessage();
message.From.Add(new MailboxAddress("Your Name", smtpUser));
message.To.Add(new MailboxAddress("Recipient", "[e-mail]"));
message.Subject = "Email with Attachment";

var body = new TextPart("plain") { Text = "Please find the attached file." };
var attachment = new MimePart("application", "pdf")
{
    Content = new MimeContent(File.OpenRead("document.pdf"), ContentEncoding.Default),
    ContentDisposition = new ContentDisposition(ContentDisposition.Attachment),
    ContentTransferEncoding = ContentEncoding.Base64,
    FileName = "document.pdf"
};

var multipart = new Multipart("mixed") { body, attachment };
message.Body = multipart;

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📧 Sending HTML Emails

message.Body = new TextPart("html")
{
    Text = "<h1>Welcome!</h1><p>This is a <strong>test email</strong> with HTML formatting.</p>"
};

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📩 Sending Emails to Multiple Recipients

message.To.Add(new MailboxAddress("Recipient1", "[e-mail]"));
message.To.Add(new MailboxAddress("Recipient2", "[e-mail]"));
message.Bcc.Add(new MailboxAddress("Hidden Recipient", "[e-mail]"));

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🔐 Using OAuth2 Authentication (Recommended for Gmail, Outlook)

using MailKit.Security;
client.Authenticate(new SaslMechanismOAuth2(smtpUser, "your-access-token"));

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📊 Implementing Logging with Serilog

using Serilog;
Log.Logger = new LoggerConfiguration()
    .WriteTo.Console()
    .WriteTo.File("logs/email.log", rollingInterval: RollingInterval.Day)
    .CreateLogger();

Log.Information("Email sent successfully to {Recipient}", "[e-mail]");

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🔄 Enhancing Resilience with Retry Policies

Using Polly to implement retries on transient errors:

var retryPolicy = Policy.Handle<SmtpCommandException>()
    .WaitAndRetry(3, retryAttempt => TimeSpan.FromSeconds(retryAttempt));

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⚡ Sending Emails Asynchronously

For better performance in web applications:

await client.SendAsync(message);
await client.DisconnectAsync(true);

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⚙ Background Email Processing

For large-scale applications, consider:

• 📅 Hangfire (Task scheduling)
• ⏳ Quartz.NET (Job scheduling)
• 📬 RabbitMQ / Azure Queue Storage (Message queuing)

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🛠 Testing Email Sending with Smtp4Dev or Mailtrap

Option 1: Smtp4Dev (Local SMTP Server)

dotnet tool install -g Rnwood.Smtp4dev
smtp4dev

Use localhost as your SMTP server in testing.

Option 2: Mailtrap (Online Testing)

• Sign up at mailtrap.io
• Use the provided SMTP credentials in your appsettings.json or environment variables.

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🏁 Conclusion

MailKit is the recommended way to send emails in .NET 9, replacing SmtpClient. It provides a modern and secure approach to email management.

🔹 Best Practices: Use OAuth2 authentication, logging with Serilog, retry policies with Polly, and test with Smtp4Dev/Mailtrap to create a robust email-sending system.