Drupal - 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:

$to  = '[email protected],[email protected],[email protected]';
drupal_mail('$module_name', $key, $to, language_default(), $params = array('username' => 'Tanvir'), $from = NULL, $send = TRUE);

Se also API documentation for drupal_mail