"\n\r" produces 2 new lines while "\n","\r" & "\r\n" produce single lines if, in the Header, you use content-type: text/plain.
Beware: If you do the Following php code:
$message='ab<br>cd<br>e<br>f';
print $message.'<br><br>';
$message=str_replace('<br>',"\r\n",$message);
print $message;
you get the following in the Windows browser:
ab
cd
e
f
ab cd e f
and with content-type: text/plain you get the following in an email output;
ab
cd
e
f