Untitled

private void WelcomeMail(string recipient)
{
    MailMessage mailMessage = new MailMessage("MyEmail", recipient);
    StringBuilder sbEmailBody = new StringBuilder();

    sbEmailBody.Append("How can I attach .html file here instead of writing the whole code");

    mailMessage.IsBodyHtml = true;
    mailMessage.Body = sbEmailBody.ToString();
    mailMessage.Subject = "Welcome to domain.com";
    SmtpClient smtpClient = new SmtpClient("smtp.gmail.com", 587);
    smtpClient.Credentials = new System.Net.NetworkCredential()
    {
        UserName = "MyEmail",
        Password = "MyPassword"
    };
    smtpClient.EnableSsl = true;
    smtpClient.Send(mailMessage);
}

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<body style="font-family: 'Open Sans', sans-serif;">
<div style="padding: 30px 30px 20px 30px;">
    <div style="padding: 30px 20px; background: #fff; font-size: 14px; color: #555; border-radius: 4px;">
        <p class="greeting" style="margin: 0;">Hello {UserName},</p>
        <p class="message" style="margin: 20px 0;">
            {Message}
        </p>
    </div>
</div>
</body>
</html>

string body = string.Empty;
string UserName = string.Empty;
string message = "This is a test email.";
StreamReader Reader = null;
WebRequest WebRequest = null;
WebResponse WebResponse = null;

WebRequest = WebRequest.Create("your_html_template_filePath");
WebRequest.Timeout = 30 * 60 * 1000;
WebRequest.UseDefaultCredentials = true;
WebResponse = _WebRequest.GetResponse();
Reader = new StreamReader(WebResponse.GetResponseStream());

body = _Reader.ReadToEnd;
body = body.Replace("{UserName}", UserName);
body = body.Replace("{Message}", message);
Reader.Close();

if (_Reader != null)
 {
     _Reader.Dispose();
 }