BCC For drupal mail function I am using Drupal 7 and focused to work with email option. I am using the Forward module. How can I add the BCC field in the drupal_mail() function.

my default function having, 

drupal_mail('forward', 'forward_page', trim($to), language_default(), $params, $params['from']);
Everything you need is in the email message headers array.

$params['headers'] = array(
    'Bcc' =&gt; 'bcc_email@example.com',
    'Cc' =&gt; 'cc_email@example.com',
);


Here is an example implementation of drupal_mail() with bcc headers included.

$params = array(
    'body' =&gt; $body,
    'subject' =&gt; $subject,
    'headers' =&gt; array(
        'Bcc' =&gt; $header_bcc,
        'Cc' =&gt; $header_cc
    )
);

$email = drupal_mail('ModuleName', 'message_key', $to, LANGUAGE_NONE, $params, $from, true);


Using hook_mail() you need to add (thank you @clive): 

/**
 * Implements hook_mail().
 */
function ModuleName_mail($key, &amp;$message, $params) {
    switch ($key) {
        case 'message_key':
            $message['headers'] += $params['headers'];
    }
}