[Dovecot] Vacation -- reply to another address than envelope from

Vacation (Sieve) replies to the envelope address. However, I have a customer which receive e-mails from a service which sends e-mails on behalf of a user (submitted through a web form).

So an e-mail would be like (envelope from and from-header is webformfromwhatever@foobar.com):

From: webformfromwhatever@foobar.com To: mycustomer@hisdomain.com Reply-To: usersrealmailaddress@example.com

I need the reply to go to the reply-to address.

Can I handle this with Sieve?

-- Vegard Svanberg vegard@svanberg.no [*Takapa@IRC (EFnet)]