problem with setting up vacation with sieve

[Marcus Schopen]

Hi,

I'm using cyrus-imapd 2.1.13-0 backport on Debian woody with the 
following vacation configuration (sieve):

########################################

require "fileinto";
require "vacation";
require "envelope";

if header :contains "subject" "vactest" {
         vacation :days 7 :addresses ["me at home.de"] "vactest!";
}
########################################


Maillog shows the following errors when I send a mail from a valid mail 
account to my email address:

Aug  2 23:13:30 dude sendmail[21513]: h72LDUWw021513: 
Authentication-Warning: dude.home.de: cyrus set sender to <> using -f
[...]
Aug  2 23:13:30 dude sendmail[21513]: h72LDUWw021513: 
to=cyrus at unspecified-domain, delay=00:00:00, xdelay=00:00:00, 
mailer=relay, pri=30388, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, 
stat=Sent (h72LDUtb021515 Message accepted for delivery)
[...]
Aug  2 23:13:30 dude sm-mta[21523]: h72LDUtb021515: 
to=<cyrus at unspecified-domain>, delay=00:00:00, xdelay=00:00:00, 
mailer=esmtp, pri=120627, relay=unspecified-domain, dsn=5.1.2, stat=Host 
unknown (Name server: unspecified-domain: host not found)
[...]
Aug  2 23:13:30 dude sm-mta[21523]: h72LDUtb021515: h72LDUta021523: 
postmaster notify: Host unknown (Name server: unspecified-domain: host 
not found)

Any idea where what is misconfigured?

Thanks,
Marcus