special-mail-tags

No. It doesn’t work.

ok, but i have a lot of extra ‘_post_xxxx’, like:

elseif ( '_post_omschrijving' == $name )
     $output = get_post_meta($post->ID, 'omschrijving', true);

How can i transform these to te newest version?

Use wpcf7_special_mail_tags filter.

But now the special-mail-tags.php no longer available is.
Where can I insert these filters?
Which file should I use?

Anywhere you like.