If create the body property as
System.Net.Mail.MailMessage message = new System.Net.Mail.MailMessage(); message.Body =\"First Line \\n second line\";
Today I found the same issue on a Error reporting app. I don't want to resort to HTML, to allow outlook to display the messages I had to do (assuming StringBuilder sb):
sb.Append(" \r\n\r\n").Append("Exception Time:" + DateTime.UtcNow.ToString());