Send email with line breaks using mail() in php
It's \r\n, as in backslash not forward slash.
Also you can try it like this:
$message = "
Hi!
This is one line.
And this is another.
Bye!
";
you should set the content type of the mail (read: tell php you're sending a html e-mail) http://php.net/manual/en/function.mail.php
it's all explained in Example #5 Sending HTML email.