System.Net.Mail.SmtpClient deletes some of my html and css tags

I wanna send a mail which formatted as html. I can do that but the mail comes wrong. When I open the source of the mail i saw that some of my styles and tags are removed but i do not know why. For example;

This is I want to send:

<div class="root-container">
    <div class="section">
        ...
    </div>
<div>

This is I received:

<div class="section">
    ...
</div>

This is my css style:

div.root-container {
        padding: 1rem;
        position: relative;
        background: linear-gradient(to right, #13a455, #0a5ca4);
        padding: 5px;
    }

This is received style:

.ROOT-CONTAINER {
    padding: 1rem;
    background: linear-gradient(to right, #13a455, #0a5ca4);
 }

C# Code:

var mail = new MailMessage();
mail.From = new MailAddress(accountContract.FromAdress, accountContract.FromName);
mail.Subject = messageContract.Subject ?? "";
mail.IsBodyHtml = true;
mail.Body = messageContract.Body;

using (var smtp = new SmtpClient())
{
    try
    {
        smtp.Credentials = new NetworkCredential(accountContract.MailUser, accountContract.MailPassword);
        smtp.Host = accountContract.Server;
        smtp.Send(mail);
    }
    catch (SmtpException ex)
    {
        WriteLog(messageContract.MessageId, "SmtpClient", null, ex.ToString());
        return MailStatus.UnProcessed;
     }                   
}

I am trouble with that problem for days. I would be really appreciated it if you could help me.