Three things.

1. As adam said :

$to = 'blah@blah.ext' . ', ';
$to = 'blah@blah.ext' . ', ';
$to = 'blah@blah.ext' . ', ';
$to .= 'blah@blah.ext';
should be
$to = 'blah@blah.ext' . ', ';
$to .= 'blah@blah.ext' . ', ';
$to .= 'blah@blah.ext' . ', ';
$to .= 'blah@blah.ext';

2. From variable breakup
Instead of using
$headers = "From: Bill@Microsoft.com";
with
mail($to, $subject, $body, $headers);

you should use (for more flexibility)
$from = "Bill.Gates@M$hit.com";
mail($to, $subject, $body, From: $headers);

The variable name 'headers' is mis-leading as a header can contain any amount of information from a vast range of categories ;).

3. Pretty nice. With a few additions and some post comments from another page, you can make a Mass E-mailer

- I decided to write a E-Mail Script. If you use it then please give credit to Omar Al-Kurd (me).
- The script i wrote is 1 page, name can be anything and it is self-sufficient/maintaing.
- If people like it alot, i may evolve it into a mailing list type thing ;).

email.php