Just to follow on from WeedPacket's post, if your message contains <br> or <br /> HTML tags where you actually want to use newline characters (to use in plain text emails), just do a simple string replace:-
$msg = "My email message!<br>This contains HTML line breaks<br>that need converting to ASCII newline charcters.<br>";
$msg = str_replace("<br>","\n",$msg);
$msg = str_replace("<br />","\n",$msg);