In a plain text email, '<br />' doesn't work. You need to replace that with "\r\n". Note the double quotes. They are required to use the escape sequences for cr-lf.
And this: ." "\n" " won't work right. You don't need the extra double quotes, just use \r\n in the string since it is already a double quoted string.
http://us.php.net/manual/en/language.types.string.php