Send Grid SMTPAPI implementation in C#

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SendGrid can update wiki docs now:

To implement Sendgrid's X-SMTPAPI in C#, we can use the MailMessage class and add headers in it.
See my example:

string headers = "{\"to\":[\"example1@example.com\",\"example2@example.com\",\"example3@example.com\"],\"filters\" : {\"domainkeys\" : {\"settings\" : {\"enable\" : 0}}}}";
//The bellow commented line works for twitter status messages
//string headers = "{\"to\":[\"example@twitterstatus\"],\"filters\" : {\"twitter\" : {\"settings\" : {\"enable\" : \"1\", \"username\" : \"example@gmail.com\",\"password\" : \"password\"}}}}";
MailMessage msg = new MailMessage();
msg.Subject = "Testing Send Grid";
msg.Body = "hello Testing via Send Grid. Its working for twitter also.. Yeppieee!!!!";
msg.From = new MailAddress("alerts@example.com");
msg.To.Add("example@example.com");
msg.Headers.Add("X-SMTPAPI", headers);
msg.IsBodyHtml = true;
System.Net.Mail.SmtpClient client = new System.Net.Mail.SmtpClient();
NetworkCredential basicauthenticationinfo = new NetworkCredential("sendgriduser@example.com", "sendgridpassword");
client.Host = "smtp.sendgrid.net";
client.Port = 25;

client.UseDefaultCredentials = false;
client.Credentials = basicauthenticationinfo;
client.DeliveryMethod = SmtpDeliveryMethod.Network;

client.Send(msg);

Send grid guys say something about RFC 821 in their api description, but even with long header strings it is working fine. I am unsure on this issue.
Also Note: Sendgrid automatically avoids sending duplicate mails, but counts them in the number of mails sent.