If create the body property as
System.Net.Mail.MailMessage message = new System.Net.Mail.MailMessage(); message.Body =\"First Line \\n second line\";
Adding . before \r\n makes it work if the original string before \r\n has no .
.
\r\n
Other characters may work. I didn't try.
With or without the three lines including IsBodyHtml, not a matter.