Sending Professional Emails in .NET: Beyond the Obsolete SmtpClient

Why SmtpClient Is No Longer Viable

Microsoft's documentation explicitly marks SmtpClient as obsolete for new development. Key limitations include:

• Lacks support for modern authentication protocols
• Missing critical security features required by contemporary email providers
• Inadequate handling of MIME formatting standards

The Modern Alternative: MailKit + MimeKit

These .NET Foundation-backed MIT-licensed libraries provide:

Implementing Email in .NET: Step-by-Step

1. Project Setup

dotnet new console -o MailDemo
cd MailDemo
dotnet add package MailKit

2. Core Message Construction

using MimeKit;
using MailKit.Net.Smtp;

var message = new MimeMessage();
message.From.Add(new MailboxAddress("Sender", "sender@domain.com"));
message.To.Add(new MailboxAddress("Recipient", "recipient@domain.com"));
message.Subject = "Email Subject";

// Plain text body
message.Body = new TextPart(TextFormat.Plain) { 
  Text = "This is a plain text email body" 
};

3. Sending with MailKit

using var client = new SmtpClient();
await client.ConnectAsync("smtp.server.com", 587, SecureSocketOptions.StartTls);
await client.AuthenticateAsync("username", "password");
await client.SendAsync(message);
await client.DisconnectAsync(true);

Advanced MIME Techniques

Multi-part Messages

Simultaneous plain text and HTML versions:

var builder = new BodyBuilder();
builder.TextBody = "Plain text content";
builder.HtmlBody = "<p>HTML content</p>";
message.Body = builder.ToMessageBody();

Attachment Handling

builder.Attachments.Add("filepath.pdf");
// For inline images:
var image = builder.LinkedResources.Add("image.jpg");
image.ContentId = MimeUtils.GenerateMessageId();
builder.HtmlBody = $@"<img src='cid:{image.ContentId}'>";

Testing with MailPit

await client.ConnectAsync("localhost", 1025);

Production Considerations

• Templating: Use Razor or MJML for maintainable HTML emails
• Background Processing: Offload sending to background services
• Security: Implement SPF, DKIM, and DMARC records
• Testing: Unit test email content generation logic