using SendGrid.CSharp.HTTP.Client;

using SendGrid.CSharp.HTTP.Client;

client.Host = "smtp.sendgrid.net";
client.Timeout = 10000;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.UseDefaultCredentials = false;
String sendgrid_username = Environment.GetEnvironmentVariable("SENDGRID_USERNAME", EnvironmentVariableTarget.User);
String sendgrid_password = Environment.GetEnvironmentVariable("SENDGRID_PASSWORD", EnvironmentVariableTarget.User);
client.Credentials = new System.Net.NetworkCredential(sendgrid_username,sendgrid_password);

MailMessage mail = new MailMessage();
mail.To.Add(new MailAddress("[email protected]"));
mail.From = "[email protected]";
mail.Subject = "this is a test email.";
mail.Body = "this is my test email body";

// add the custom header that we built above
mail.Headers.Add( "X-SMTPAPI", xmstpapiJson );

client.SendAsync(mail, null);

var header = new Header();

var recipients = new List<String> {"[email protected]", "[email protected]", "[email protected]"};
header.SetTo(recipients);

var subs = new List<String> {"A","B","C"};
header.AddSubstitution("%name%", subs);

var mail = new MailMessage
{
    From = new MailAddress("[email protected]"),
    Subject = "Welcome",
    Body = "Hi there %name%"
};

// add the custom header that we built above
mail.Headers.Add("X-SMTPAPI", header.JsonString());