[exim] user forward and smarthost

Hello,

I use debian and exim with a smarthost. The mailing works fine.

Now I want to deliver some mails to another external address. So I
used the deliver-command in the .forward file. But after that, the
mails become frozen, because the Smarthosts does not accept the
messages. The envelope is empty:

if $header_from: contains "mail@???"
then
deliver "Mail User <mail.remote@???>"
endif

SMTP error from remote mail server after MAIL FROM:<> SIZE=4331: host
mail.gmx.net [213.165.64.21]: 550 5.1.7 This server does not accept an
empty envelope from ( http://portal.gmx.net/serverrules )

Does anybody have a hint for me. What do I have to change?

Thanks a lot
Oliver