MimeKit and MailKit

Overview

MimeKit is a .NET library for creating and parsing MIME messages (the format underlying email). MailKit is the companion transport library that sends messages via SMTP and retrieves them via IMAP and POP3. Together, they replace the deprecated System.Net.Mail with a modern, standards-compliant, cross-platform solution. MimeKit supports complex MIME structures including multipart messages, inline images, file attachments, S/MIME encryption, and PGP/MIME signing.

MailKit provides full support for OAuth2 authentication, STARTTLS, connection pooling, and progress reporting, making it suitable for both simple transactional email and advanced mail client applications.

Sending a Simple Email

Create a message and send it via SMTP using MailKit.

csharp

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

public class EmailService
{
    public async Task SendSimpleEmailAsync()
    {
        var message = new MimeMessage();
        message.From.Add(new MailboxAddress(
            "Order System", "orders@example.com"));
        message.To.Add(new MailboxAddress(
            "John Doe", "john@example.com"));
        message.Subject = "Your order has shipped";

        message.Body = new TextPart("plain")
        {
            Text = "Your order #12345 has shipped "
                + "and will arrive in 3-5 business days."
        };

        using var client = new SmtpClient();
        await client.ConnectAsync(
            "smtp.example.com", 587,
            SecureSocketOptions.StartTls);
        await client.AuthenticateAsync(
            "smtp-user", "smtp-password");
        await client.SendAsync(message);
        await client.DisconnectAsync(quit: true);
    }
}

HTML Email with Inline Images

Build multipart messages with both plain text and HTML alternatives, plus inline images.

csharp

using MimeKit;
using MimeKit.Utils;
using System.IO;

public static class HtmlEmailBuilder
{
    public static MimeMessage CreateHtmlEmail(
        string toName, string toEmail,
        string subject, string htmlBody, string textBody)
    {
        var message = new MimeMessage();
        message.From.Add(new MailboxAddress(
            "MyApp", "noreply@example.com"));
        message.To.Add(new MailboxAddress(toName, toEmail));
        message.Subject = subject;

        var builder = new BodyBuilder
        {
            TextBody = textBody,
            HtmlBody = htmlBody
        };

        // Inline image referenced in HTML as cid:logo
        var image = builder.LinkedResources.Add("images/logo.png");
        image.ContentId = MimeUtils.GenerateMessageId();
        builder.HtmlBody = htmlBody.Replace(
            "{{logo_cid}}", image.ContentId);

        message.Body = builder.ToMessageBody();
        return message;
    }
}

File Attachments

Attach files with proper MIME types and content disposition.

csharp

using MimeKit;
using System.IO;

public static class AttachmentEmailBuilder
{
    public static MimeMessage CreateWithAttachments(
        string to, string subject,
        string body, params string[] filePaths)
    {
        var message = new MimeMessage();
        message.From.Add(new MailboxAddress(
            "Reports", "reports@example.com"));
        message.To.Add(MailboxAddress.Parse(to));
        message.Subject = subject;

        var builder = new BodyBuilder
        {
            HtmlBody = body
        };

        foreach (var path in filePaths)
        {
            builder.Attachments.Add(path);
        }

        message.Body = builder.ToMessageBody();
        return message;
    }

    public static MimeMessage CreateWithStreamAttachment(
        string to, string subject, string body,
        string fileName, Stream content, string mimeType)
    {
        var message = new MimeMessage();
        message.From.Add(new MailboxAddress(
            "Reports", "reports@example.com"));
        message.To.Add(MailboxAddress.Parse(to));
        message.Subject = subject;

        var builder = new BodyBuilder
        {
            HtmlBody = body
        };

        builder.Attachments.Add(fileName, content,
            ContentType.Parse(mimeType));

        message.Body = builder.ToMessageBody();
        return message;
    }
}

Reading Email via IMAP

Use MailKit's IMAP client to retrieve and process emails.

csharp

using MailKit;
using MailKit.Net.Imap;
using MailKit.Search;
using MailKit.Security;
using MimeKit;
using System;
using System.Collections.Generic;
using System.Threading.Tasks;

public class InboxReader
{
    public async Task<List<MimeMessage>> GetUnreadMessagesAsync(
        string host, int port,
        string username, string password)
    {
        var messages = new List<MimeMessage>();

        using var client = new ImapClient();
        await client.ConnectAsync(host, port,
            SecureSocketOptions.SslOnConnect);
        await client.AuthenticateAsync(username, password);

        var inbox = client.Inbox;
        await inbox.OpenAsync(FolderAccess.ReadWrite);

        var uids = await inbox.SearchAsync(SearchQuery.NotSeen);

        foreach (var uid in uids)
        {
            var message = await inbox.GetMessageAsync(uid);
            messages.Add(message);

            // Mark as read
            await inbox.AddFlagsAsync(uid,
                MessageFlags.Seen, silent: true);
        }

        await client.DisconnectAsync(quit: true);
        return messages;
    }
}

ASP.NET Core Integration

csharp

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Options;
using MailKit.Net.Smtp;
using MailKit.Security;
using MimeKit;
using System.Threading;
using System.Threading.Tasks;

public class SmtpSettings
{
    public string Host { get; set; } = string.Empty;
    public int Port { get; set; } = 587;
    public string Username { get; set; } = string.Empty;
    public string Password { get; set; } = string.Empty;
    public string FromName { get; set; } = string.Empty;
    public string FromEmail { get; set; } = string.Empty;
}

public class SmtpEmailSender
{
    private readonly SmtpSettings _settings;

    public SmtpEmailSender(IOptions<SmtpSettings> settings)
    {
        _settings = settings.Value;
    }

    public async Task SendAsync(
        MimeMessage message, CancellationToken ct = default)
    {
        if (message.From.Count == 0)
        {
            message.From.Add(new MailboxAddress(
                _settings.FromName, _settings.FromEmail));
        }

        using var client = new SmtpClient();
        await client.ConnectAsync(
            _settings.Host, _settings.Port,
            SecureSocketOptions.StartTls, ct);
        await client.AuthenticateAsync(
            _settings.Username, _settings.Password, ct);
        await client.SendAsync(message, ct);
        await client.DisconnectAsync(quit: true, ct);
    }
}

// Registration
// builder.Services.Configure<SmtpSettings>(
//     builder.Configuration.GetSection("Smtp"));
// builder.Services.AddTransient<SmtpEmailSender>();

MailKit Transport Comparison

Protocol	Class	Port	Use Case
SMTP + STARTTLS	`SmtpClient`	587	Sending email (recommended)
SMTP + SSL	`SmtpClient`	465	Sending email (legacy)
IMAP + SSL	`ImapClient`	993	Reading email (full-featured)
POP3 + SSL	`Pop3Client`	995	Reading email (download-and-delete)

Best Practices

•Use MailKit instead of System.Net.Mail because SmtpClient in System.Net.Mail is deprecated, does not support modern authentication (OAuth2), and has known security issues.
•Always use SecureSocketOptions.StartTls or SslOnConnect when connecting to mail servers; never use SecureSocketOptions.None in production to prevent credential interception.
•Dispose SmtpClient after each send (via using) rather than keeping a long-lived connection; SMTP connections are lightweight and servers may time out idle connections.
•Use BodyBuilder to construct multipart messages with both text and HTML bodies, so email clients without HTML support fall back to the plain text version.
•Set ContentId on inline images using MimeUtils.GenerateMessageId() and reference them in HTML via cid: URLs for reliable rendering across email clients.
•Store SMTP credentials in configuration (appsettings.json or environment variables) via the options pattern, never hard-code them in source files.
•Validate email addresses using MailboxAddress.TryParse() before constructing messages to catch formatting errors early and avoid SmtpClient exceptions.
•Handle transient SMTP failures (connection timeouts, temporary 4xx errors) with a retry policy or background queue rather than failing the user's request synchronously.
•Use IMAP IDLE (ImapClient.IdleAsync) instead of polling when building a mail-monitoring service to receive real-time notifications of new messages without repeated searches.
•Set message.MessageId explicitly using MimeUtils.GenerateMessageId() for traceability and to prevent duplicate detection issues in recipient mail servers.