Setting isBodyHtml to true allows you to use HTML tags in the message body:
msg = new MailMessage("[email protected]",
                "[email protected]", "Message from PSSP System",
                "This email sent by the PSSP system<br />" +
                "<b>this is bold text!</b>");
msg.IsBodyHtml = true;
    Best way to send html formatted Email
This code will be in "Customer.htm"
    <table>
    <tr>
        <td>
            Dealer's Company Name
        </td>
        <td>
            :
        </td>
        <td>
            #DealerCompanyName#
        </td>
    </tr>
</table>
Read HTML file Using System.IO.File.ReadAllText. get all HTML code in string variable.
string Body = System.IO.File.ReadAllText(HttpContext.Current.Server.MapPath("EmailTemplates/Customer.htm"));
Replace Particular string to your custom value.
Body = Body.Replace("#DealerCompanyName#", _lstGetDealerRoleAndContactInfoByCompanyIDResult[0].CompanyName);
call SendEmail(string Body) Function and do procedure to send email.
 public static void SendEmail(string Body)
        {
            MailMessage message = new MailMessage();
            message.From = new MailAddress(Session["Email"].Tostring());
            message.To.Add(ConfigurationSettings.AppSettings["RequesEmail"].ToString());
            message.Subject = "Request from " + SessionFactory.CurrentCompany.CompanyName + " to add a new supplier";
            message.IsBodyHtml = true;
            message.Body = Body;
            SmtpClient smtpClient = new SmtpClient();
            smtpClient.UseDefaultCredentials = true;
            smtpClient.Host = ConfigurationSettings.AppSettings["SMTP"].ToString();
            smtpClient.Port = Convert.ToInt32(ConfigurationSettings.AppSettings["PORT"].ToString());
            smtpClient.EnableSsl = true;
            smtpClient.Credentials = new System.Net.NetworkCredential(ConfigurationSettings.AppSettings["USERNAME"].ToString(), ConfigurationSettings.AppSettings["PASSWORD"].ToString());
            smtpClient.Send(message);
        }
    This works for me
msg.BodyFormat = MailFormat.Html;
and then you can use html in your body
msg.Body = "<em>It's great to use HTML in mail!!</em>"
    
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