📌 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 In .NET 9, the SmtpClient class is obsolete and no longer recommended.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.
✅ 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)
🔧 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=your-email@example.com
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");
📤 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", "recipient@example.com"));
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}");
}
}
}
🔒 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. |
📎 Sending Emails with Attachments
var message = new MimeMessage();
message.From.Add(new MailboxAddress("Your Name", smtpUser));
message.To.Add(new MailboxAddress("Recipient", "recipient@example.com"));
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;
📧 Sending HTML Emails
message.Body = new TextPart("html")
{
Text = "<h1>Welcome!</h1><p>This is a <strong>test email</strong> with HTML formatting.</p>"
};
📩 Sending Emails to Multiple Recipients
message.To.Add(new MailboxAddress("Recipient1", "recipient1@example.com"));
message.To.Add(new MailboxAddress("Recipient2", "recipient2@example.com"));
message.Bcc.Add(new MailboxAddress("Hidden Recipient", "bcc@example.com"));
🔐 Using OAuth2 Authentication (Recommended for Gmail, Outlook)
using MailKit.Security;
client.Authenticate(new SaslMechanismOAuth2(smtpUser, "your-access-token"));
📊 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}", "recipient@example.com");
🔄 Enhancing Resilience with Retry Policies
Using Polly to implement retries on transient errors:
var retryPolicy = Policy.Handle<SmtpCommandException>()
.WaitAndRetry(3, retryAttempt => TimeSpan.FromSeconds(retryAttempt));
⚡ Sending Emails Asynchronously
For better performance in web applications:
await client.SendAsync(message);
await client.DisconnectAsync(true);
⚙ Background Email Processing
For large-scale applications, consider:
- 📅 Hangfire (Task scheduling)
- ⏳ Quartz.NET (Job scheduling)
- 📬 RabbitMQ / Azure Queue Storage (Message queuing)
🛠 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.jsonor environment variables.
🏁 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.
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