Update post meta array – add new, single value

I think you’re close. In my code I’ve assumed that the desired customer_email meta looks like this:

array(
    0 => '[email protected]',
    1 => '[email protected]',
    2 => '[email protected]',
);

What you need to do is add the new email address to the array of existing email addresses, then save the updated array back to the meta value. update_post_meta() simply overwrites whatever’s already in the meta value, so you can’t use it to simply merge a new value into an existing array.

Something like this should do it:

add_action('wpcf7_mail_sent',
    function ($contact_form) {
        if ($contact_form->id() === 32009) {
            $submission = WPCF7_Submission::get_instance();
            $currentdata = get_post_meta(32127, 'customer_email', true);
            if ( empty( $currentdata ) ) {
                $currentdata = array();
            }
            // Bail out if $currentdata *isn't* an array.
            if ( ! is_array( $currentdata ) ) {
                return;
            }
            $mail = $submission->get_posted_data('email-002');
            // Add the new email address to $currentdata.
            $currentdata[] = $mail;
            update_post_meta(32127, 'customer_email',$currentdata);
        }
    }
);

Edited: I’ve changed get_post_meta( 32127, 'customer_email' ); to get_post_meta( 32127, 'customer_email', true ); to ensure that we only get a single post_meta value. Otherwise it will grab all the post_meta values with the key customer_email.

I’m not sure that’s the cause of the error you’re seeing in the comments, but it should make the code more precise.