[exim] How I do to add headers to a unseen save in a system …

I wonder, how I do to add headers to a unseen save in a system filter, but
which is NOT sent through to the original recipient, just used for the
unseen save?

I tried with:

if $received_port is 10026 then
if $sender_address_domain is sebbe.eu then
headers add "Status: RO"
unseen save /var/mail/sebastian/Sent
headers remove "Status"
endif
endif

but didn't work.
To make sure it was not Dovecot changing the Status: header line, I tried
with:

if $received_port is 10026 then
if $sender_address_domain is sebbe.eu then
headers add "X-Weird-Header: RO"
unseen save /var/mail/sebastian/Sent
headers remove "X-Weird-Header"
endif
endif

but X-Weird-Header does not appear in the saved copy.

Best regards, Sebastian Nielsen