[exim] How to set autoreply in exim ?

Hello,

I would like to send autoreplay message to anybody would send email to
virtualdomain1.com

I have put to exim filer

if $h_to: matches "virtualdomain1.com" then
    mail
    to $reply_address
    subject "Re: $h_subject"
    text "Thank you for emailing I'm on the vacation"
    log
    once
    once_repeat 7d
endif

Unfortunately autoreply message is not send and I'm having following
error added to my logs:
2006-10-12 14:10:15 1GY4eB-0004ou-Ed =3D=3D >"Slawomir Orlowski
\(Cympak\)" <sorlowski@???> <system-filter> routing defer (-1):
system_filter_reply_transport is unset

How do I set system_filter_reply_transport ?

regards
I would appreciate any help.
Slawomir