Why MailKit Is the Best Choice for Sending Emails in .NET Core

Step-by-Step Guide to Implementing Efficient Email Sending in .NET Core with MailKit

Sending emails is a common requirement in modern applications — whether for account verification, password resets, notifications, or reports. As a .NET developer, you may have encountered the built-in System.Net.Mail.SmtpClient class in your projects. It works… but it comes with several limitations.

Enter MailKit — a modern, robust, and high-performance alternative that every .NET developer should know about.

📚 The Legacy Problem: SmtpClient

The System.Net.Mail.SmtpClient was introduced back in the .NET Framework days. It is simple and convenient for sending a few emails here and there. However, it was never designed for high-performance applications.

⚠️ Key Limitations of SmtpClient:

• Poor async implementation
  Async support exists, but it is built on top of blocking threads, resulting in poor scalability.

• Inefficient connection handling
  Every time you send an email, the connection is established, and closed. No connection pooling or reuse, which is a big performance hit when sending multiple emails.

• Higher CPU & memory usage
  Especially noticeable when sending emails in bulk or under concurrent loads.

👉 Microsoft now considers it legacy, and it’s not recommended for .NET Core or .NET 6+ apps.

🌟 The Modern Solution: MailKit

MailKit is a fully-featured, open-source mail client library built for .NET. It’s designed for modern apps with performance, security, and scalability in mind.

✅ Why MailKit Rocks:

• Truly Async API
  Designed from the ground up to work seamlessly with async/await, enabling efficient non-blocking I/O.

• Connection Reuse & Pooling
  Reuse SMTP connections when sending multiple emails. Perfect for background workers or SaaS apps that send bulk emails.

• Cross-Platform Support
  Works on Windows, Linux, and macOS without any platform-specific quirks.

• Advanced Features
  Easily send HTML emails, attachments, inline images, and more. Supports OAuth2 and modern authentication.

✅ Sample MailKit Usage

1️⃣ Controller Layer

using Microsoft.AspNetCore.Mvc;
using SendEmailUsingMailKit.Services.Tempest.Services;

namespace SendEmailUsingMailKit.Controllers
{
    [ApiController]
    [Route("[controller]")]
    public class EmailController : ControllerBase
    {
        private readonly IMailer _mailer;

        public EmailController(IMailer mailer)
        {
            _mailer = mailer;
        }

        [HttpGet]
        [Route("export")]
        public async Task<IActionResult> ExportWeatherReport()
        {
            await _mailer.SendEmailAsync("temp-email@gmail.com", "Weather Report", "Detailed Weather Report");
            return NoContent();
        }
    }
}

2️⃣ SMTP Settings Entity

namespace SendEmailUsingMailKit.Entities
{
    public class SmtpSettings
    {
        public string Server { get; set; }
        public int Port { get; set; }
        public string SenderName { get; set; }
        public string SenderEmail { get; set; }
        public string Username { get; set; }
        public string Password { get; set; }
    }
}

3️⃣ Mailer Service Implementation

using MailKit.Net.Smtp;
using MailKit.Security;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Options;
using MimeKit;
using SendEmailUsingMailKit.Entities;
using System.Threading.Tasks;

namespace SendEmailUsingMailKit.Services.Tempest.Services
{
    public interface IMailer
    {
        Task SendEmailAsync(string email, string subject, string body);
    }

    public class Mailer : IMailer
    {
        private readonly SmtpSettings _smtpSettings;
        private readonly IWebHostEnvironment _env;

        public Mailer(IOptions<SmtpSettings> smtpSettings, IWebHostEnvironment env)
        {
            _smtpSettings = smtpSettings.Value;
            _env = env;
        }

        public async Task SendEmailAsync(string toEmail, string subject, string body)
        {
            try
            {
                var message = new MimeMessage();
                message.From.Add(new MailboxAddress(_smtpSettings.SenderName, _smtpSettings.SenderEmail));
                message.To.Add(MailboxAddress.Parse(toEmail));
                message.Subject = subject;
                message.Body = new TextPart("html")
                {
                    Text = body
                };

                using (var client = new SmtpClient())
                {
                    await client.ConnectAsync(_smtpSettings.Server, _smtpSettings.Port, SecureSocketOptions.StartTls);
                    await client.AuthenticateAsync(_smtpSettings.Username, _smtpSettings.Password);
                    await client.SendAsync(message);
                    await client.DisconnectAsync(true);
                }
            }
            catch (Exception e)
            {
                throw new InvalidOperationException(e.Message);
            }
        }
    }
}

4️⃣ Configuration in appsettings.json

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  },
  "AllowedHosts": "*",
  "SmtpSettings": {
    "Server": "smtp.gmail.com",
    "Port": 587,
    "SenderName": "My Name",
    "SenderEmail": "<my-account-user>@gmail.com",
    "Username": "<my-account-user>@gmail.com",
    "Password": "<my-account-password>"
  }
}

✅ Conclusion

If you’re building production-grade .NET applications that send emails, stop relying on System.Net.Mail.SmtpClient.
👉 Adopt MailKit for efficient async handling, connection reuse, scalability, and better performance.

MailKit is battle-tested and widely adopted by the .NET community. It’s the right choice for modern software architecture.

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🌟 Do you use MailKit in your projects already? Share your experience in the comments!

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👉 Full working code available at:

🔗https://sourcecode.kanaiyakatarmal.com/SendEmailUsingMailKit

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I hope you found this guide helpful and informative.

Thanks for reading!

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