envelope and virtual domain with postfix

[Alexander Steinert <stony8@gmx.de>]

Hi,

my /etc/postfix/virtual on host.bar.tld contains `@foo.tld @localhost'. 

My ISP provides webspace for foo.tld and is "configured" to forward all
mail addressed to *@foo.tld to filter@host.bar.tld.

Incoming mail to say me@foo.tld arrives at filter@host.bar.tld and
contains `me@foo.tld' only in the To: header and in exactly one
Received: from ... by ... for ... header.

How can I make postfix write `me@foo.tld' onto the envelope in order to
get the virtual domain working?

TIA

Stony