Already i done a mail gateway by using my current mail id. Now i want this mail services to done through third party services. because i want to send bulk email to my customers. I created my account in sendin blue site. if i give that smtp name means its saying its not authorised to send mail.

This is my Error

The SMTP server requires a secure connection or the client was not authenticated. The server response was: Error: authentication Required

Can you say how to send email from third party mailing tool. Moreover if i send email means it should be count from my third party email(sendinblue) not from my gmail count.

Gmail SMTP name is smtp.gmail.com

Send inbluue SMTP name is smtp-relay.sendinblue.com

dt3 = load.getMaildet();
string city = "";
string to = txtemail.Text; //To address    
string from = dt3.Rows[0][1].ToString(); //From address
string password= dt3.Rows[0][2].ToString();
string sermailid= dt3.Rows[0][3].ToString();
string serno= dt3.Rows[0][4].ToString();
MailMessage message = new MailMessage(from, to);
message.Subject = "Requested New Connection";
string text = "<a href = 'http://www.google.com' target = '_new' style = 'text-decoration:none;'><table border = '0' align='center' cellpadding = '5' cellspacing = '5' bgcolor = '#E6E6E6' style = 'padding-right:400px;padding-top:50px;padding-bottom:50px;padding-left:50px;'><tr><td><h1>Dear " + txtname.Text + " <h1><br></td></tr><tr><td><b>Thank you !</b> For Choosing our Product<b> " + ddlcontent.SelectedItem + "</b>.<br>Note this Number for future Reference <b>" + LeadID + "</b>. <br/> Once again Thanking you and assuring you of our best services always. We will contact you shortly.</td></tr><tr><td><h1><h1></td></tr><tr><td><input type = 'button' style = 'background-color:#2C3E50; color:#FFFFFF; border:0px; padding:10px;' value = 'Continue to Homepage' onclick = 'window.open('http://www.google.com')'/></td></tr></table></a>";
message.IsBodyHtml = true;
message.Body = text;
SmtpClient client = new SmtpClient(sermailid, Convert.ToInt32(serno)); //Gmail smtp    
System.Net.NetworkCredential basicCredential1 = new
System.Net.NetworkCredential(from, password);
client.EnableSsl = true;
client.UseDefaultCredentials = false;
client.Credentials = basicCredential1;
client.Send(message);