Question

How to request the operating system to send email with a specific subject, body and attachment? The OS should launch the default email client with the fields pre-loaded?

Specifically what API functions are available in C#.net?

Answer 1

Since you want the outlook to send the email, substitute the port number and host with that of the outlook.

To Open Outlook with pre-loaded fields:

using System.Diagnostics;
// ...
Process.Start(String.Format("mailto:{0}?subject={1}&cc={2}&bcc={3}&body={4}", address, subject, cc, bcc, body))

To send the Email directly :

using System.Net;
using System.Net.Mail;
// ...
var fromAddress = new MailAddress("[e-mail]", "From Name");
var toAddress = new MailAddress("[e-mail]", "To Name");
const string fromPassword = "fromPassword";
const string subject = "Subject";
const string body = "Body";

var smtp = new SmtpClient {
    Host = "smtp.gmail.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);
}