Sending Email in C# (.NET) using SmtpClient and MailKit

Sending Email in C# (.NET): SmtpClient, MailKit, and MimeKit

Sending email is one of the basic tasks in modern applications. In the C# (.NET) ecosystem, there are both built-in tools (the SmtpClient class) and powerful third-party libraries (MailKit and MimeKit). In this guide, we will thoroughly cover both approaches, review code examples, SMTP server configuration, working with attachments, and HTML emails.

1. Built-in SmtpClient (System.Net.Mail)

The SmtpClient class is available in .NET Framework and .NET Core/5+ (with limitations). It is simple to use but does not support modern protocols (e.g., OAuth2) and is considered deprecated for new projects.

2. Installing and Connecting MailKit

MailKit is a cross-platform library for working with email. It is fully asynchronous, supports SSL/TLS, OAuth2, S/MIME, and other modern features. Installation via NuGet:

dotnet add package MailKitdotnet add package MimeKit

Or via the Package Manager Console:

Install-Package MailKitInstall-Package MimeKit

3. Example of sending a simple email using SmtpClient

Basic example using the built-in SmtpClient:

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

public void SendSimpleEmail(){    var fromAddress = new MailAddress("sender@example.com", "Sender");    var toAddress = new MailAddress("recipient@example.com", "Recipient");    const string fromPassword = "your_password";    const string subject = "Email Subject";    const string body = "Email Body";

    var smtp = new SmtpClient    {        Host = "smtp.example.com",        Port = 587,        EnableSsl = true,        DeliveryMethod = SmtpDeliveryMethod.Network,        UseDefaultCredentials = false,        Credentials = new NetworkCredential(fromAddress.Address, fromPassword)    };

    using (var message = new MailMessage(fromAddress, toAddress)    {        Subject = subject,        Body = body    })    {        smtp.Send(message);    }}

Important: For Gmail, Yandex, and other services, you need to use an app password or OAuth2.

4. Sending Email using MailKit (recommended method)

MailKit provides a more flexible and modern API. Example of sending a simple text email:

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

public async Task SendEmailWithMailKitAsync(){    var message = new MimeMessage();    message.From.Add(new MailboxAddress("Sender", "sender@example.com"));    message.To.Add(new MailboxAddress("Recipient", "recipient@example.com"));    message.Subject = "Email Subject";    message.Body = new TextPart("plain")    {        Text = "Email Body"    };

    using (var client = new SmtpClient())    {        await client.ConnectAsync("smtp.example.com", 587, SecureSocketOptions.StartTls);        await client.AuthenticateAsync("sender@example.com", "your_password");        await client.SendAsync(message);        await client.DisconnectAsync(true);    }}

5. Sending an HTML email with attachments

Creating a multipart email (HTML + attachments) using MimeKit:

public async Task SendHtmlEmailWithAttachmentAsync(){    var message = new MimeMessage();    message.From.Add(new MailboxAddress("Sender", "sender@example.com"));    message.To.Add(new MailboxAddress("Recipient", "recipient@example.com"));    message.Subject = "HTML email with attachment";

    var bodyBuilder = new BodyBuilder    {        HtmlBody = @"<h1>Hello!</h1><p>This is an HTML email with an attachment.</p>",        TextBody = "This is a text