◆確認用にメールアドレスを２回入力させ、尚且つチェックする機能

1:まずfunction.phpに下記の記述を追加します。

<?php
/* custom contactform7 */
add_filter( 'wpcf7_validate_email', 'wpcf7_text_validation_filter_extend', 11, 2 );
add_filter( 'wpcf7_validate_email*', 'wpcf7_text_validation_filter_extend', 11, 2 );
function wpcf7_text_validation_filter_extend( $result, $tag ) {
    $type = $tag['type'];
    $name = $tag['name'];
    $_POST[$name] = trim( strtr( (string) $_POST[$name], "\n", " " ) );
    if ( 'email' == $type || 'email*' == $type ) {
        if (preg_match('/(.*)_confirm$/', $name, $matches)){
            $target_name = $matches[1];
            if ($_POST[$name] != $_POST[$target_name]) {
                $result['valid'] = false;
                $result['reason'][$name] = 'メールアドレスが一致しません';
            }
        }
    }
    return $result;
}
?>

２：次にフォームの設定では下記のように

[email* your-email]
[email* your-email_confirm]

※透かし文字を入れる場合は

[email* your-email]
[email* your-email_confirm watermark"確認のため再度ご入力ください"]