A message could be sent as "Plain Text" or "HTML".  Use <br> tag if the content is HTML and \n if the content type is "plain text".  For example, 

on  add
    {
        on success
        {
            sendmail
            (
                To       :  zoho.loginuserid 
                From     :  zoho.adminuserid 
                Subject  :  "To insert line breaks" 
                Message  :  "Dear Customer, "+ "<br>Your order has been processed"
            )
        }
    }