PHP send plain text mail and force fixed-width charset

I want to force the reader's email client to display my php-generated plain text emails with a fixed width charset. Thunderbird displays the email with a fixed width charset. However, Gmail and Outlook (and probably more clients) do not.

Is this a config setting with my mail server or something I'm doing wrong? Any help would be greatly appreciated.

These are the headers I'm currently using:

    $header = "MIME-Version: 1.0\r\n"; 
    $header .= "Content-type: text/plain开发者_StackOverflow中文版; charset=ISO-8859-1; format=flowed\r\n"; 
    $header .= "Content-Transfer-Encoding: 7bit\r\n"; 
    $header .= "X-Mailer: PHP" . phpversion() ."\r\n"; 
    $header .= "From: ihateoutlook@email.com\r\n";
    $header .= "Reply-To: ihateoutlook@email.com\r\n";

You cannot force the plain text message to use any specific font. The best you can do is send HTML email using the monospace font family.