How to send HTML or plain text message via Mimekit

Question

I'm trying to send an HTML message with my font and size but it doesn't work.

If I'm not mistaken, I have to convert the text in my richTextBox from RTF to HTML to be able to do that, I think my problem is there.

My code gives this:

                    BodyBuilder builder = new BodyBuilder();
                    MimeMessage mime = new MimeMessage();
                    if(checkBox1.Checked)
                    {
                        
                        builder.HtmlBody = richTextBox1.Rtf;
                    }
                    else
                    {
                        builder.TextBody = richTextBox1.Text;
                    }
                    
                    mime.Body = builder.ToMessageBody();


    private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
    {
         richTextBox1.Font = new Font(comboBox1.Text, richTextBox1.Font.Size);
         
    }
    private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
    {
        richTextBox1.Font = new Font(richTextBox1.Font.FontFamily, float.Parse(comboBox2.SelectedItem.ToString()));
        
    }

Thanks for your help please