mailkit_usage_notes.md

MailKit Usage Notes for Email Rewriting and Sending

Overview

MailKit, along with its dependency MimeKit, provides the necessary tools to parse, modify, and send emails in .NET.

Parsing Raw Email from Webhook Payload

The Forward Email webhook payload includes a raw field containing the full raw email source as a string. MimeKit can parse this:

1. Convert the raw email string into a stream (e.g., MemoryStream).
2. Use MimeMessage.Load to parse the stream into a MimeMessage object.

using MimeKit;
using System.IO;
using System.Text;

// Assuming 'rawEmailString' contains the value from the webhook payload's 'raw' field
byte[] rawEmailBytes = Encoding.UTF8.GetBytes(rawEmailString);
using (var stream = new MemoryStream(rawEmailBytes))
{
    // Load the message
    // Set persistent = true if you need to modify the message later
    var message = MimeMessage.Load(stream, persistent: true);

    // Now you can access message properties like headers, body, etc.
    // Example: Accessing the Subject
    // Console.WriteLine($"Subject: {message.Subject}");
}

Note: The LoadFromPickupDirectory example in SmtpExamples.cs uses SmtpDataFilter(decode: true) because pickup directory files are byte-stuffed. This filter is not needed when loading directly from the raw email string provided by the webhook, as it's not byte-stuffed in that context.

Modifying the Email (Rewriting)

Once parsed into a MimeMessage object, you can modify its properties:

• Headers: Access and modify headers via the message.Headers collection or specific properties like message.Subject, message.To, message.From, message.Cc, message.Bcc.

  // Example: Change the Subject
  message.Subject = "[Processed] " + message.Subject;
  
  // Example: Clear existing recipients and add a new one
  message.To.Clear();
  message.Cc.Clear();
  message.Bcc.Clear();
  message.To.Add(new MailboxAddress("New Recipient Name", "new.recipient@example.com"));

• Body: Access/modify the body via message.Body. This might be plain text (TextPart), HTML (HtmlPart), or multipart. Use BodyBuilder for easier construction/modification.

  // Example: Prepend text to the existing text body (if it exists)
  var textPart = message.TextBody;
  if (textPart != null)
  {
      message.Body = new TextPart("plain")
      {
          Text = "This email was processed by our webhook.\n\n" + textPart
      };
  }
  // Note: More complex modifications might involve rebuilding the message structure, especially for multipart messages.

Sending Email via SMTP

The SmtpExamples.cs file demonstrates the standard SMTP sending process using MailKit.Net.Smtp.SmtpClient:

using MailKit.Net.Smtp;
using MailKit.Security;
using MimeKit;

// Assuming 'message' is the MimeMessage object (potentially modified)
// Assuming SMTP settings (host, port, user, pass, useSsl) are configured

using (var client = new SmtpClient())
{
    try
    {
        // Determine SecureSocketOptions based on configuration (e.g., port 465 -> SslOnConnect, port 587 -> StartTls)
        var secureSocketOptions = useSsl ? SecureSocketOptions.SslOnConnect : SecureSocketOptions.StartTlsWhenAvailable;
        client.Connect(smtpHost, smtpPort, secureSocketOptions);

        // Authenticate if credentials are provided
        if (!string.IsNullOrEmpty(smtpUser))
        {
            client.Authenticate(smtpUser, smtpPass);
        }

        // Send the message
        client.Send(message);

        client.Disconnect(true);
        Console.WriteLine("Email sent successfully.");
    }
    catch (AuthenticationException ex)
    {
        Console.WriteLine($"SMTP Authentication Error: {ex.Message}");
        // Handle error (log, notify admin, etc.)
    }
    catch (SmtpCommandException ex)
    {
        Console.WriteLine($"SMTP Command Error: {ex.Message}");
        Console.WriteLine($"\tStatusCode: {ex.StatusCode}");
        // Handle error
    }
    catch (SmtpProtocolException ex)
    {
        Console.WriteLine($"SMTP Protocol Error: {ex.Message}");
        // Handle error
    }
    catch (Exception ex)
    {
        Console.WriteLine($"General Error sending email: {ex.Message}");
        // Handle error
    }
}

Key Considerations:

• Configuration: SMTP server details (host, port, credentials, SSL/TLS usage) must be configurable.
• Security: Use appropriate SecureSocketOptions (e.g., SslOnConnect for port 465, StartTls or StartTlsWhenAvailable for port 587).
• Error Handling: Implement robust error handling for connection, authentication, and sending phases.
• Logging: Use ProtocolLogger during development/debugging if needed: new SmtpClient(new ProtocolLogger("smtp.log")).

References

• MailKit GitHub: https://github.com/jstedfast/MailKit
• SMTP Examples: https://github.com/jstedfast/MailKit/blob/master/Documentation/Examples/SmtpExamples.cs
• MimeKit Documentation: (Linked from MailKit repo, likely https://mimekit.net)