How to send email to multiple recipients using drupal_mail?
The to-parameter to drupal_mail is a string, not an array.  But you can have as many receipent as you like in the the e-mails "to" string, provided they're separated by commas.

So to send the same mail to multiple recipients, do the following:

$to  = 'one@email.com,two@email.com,three@email.com';
drupal_mail('$module_name', $key, $to, language_default(), $params = array('username' =&gt; 'Tanvir'), $from = NULL, $send = TRUE);


Se also API documentation for drupal_mail