What you need to do here is build your email body using HTML syntax and set MailMessage.IsBodyHtml
property to true
before sending.
oBLL.Comments = "<strong>Comments:</strong>" + txtComments.Text;
mailBody.Append(oBLL.Comments);
// ...
// assuming mail client and message instances already exists
mailMessage.IsBodyHtml = true;
mailClient.Send(mailMessage);