different set of read articles in Gnus and in the IMAP server

[Gábor Vida]

Hi!

I have many nnimap groups in which there are articles read. When I quit
from the group, the server is asked to provide the list of expirable
articles using e.g. "UID SEARCH SENTBEFORE 23-SEPT-2011" command, but
the server doesn't reply back some old articles that are already read
according to Gnus, and thus they won't be treated as expired.

Could you please help me how can I force those articles to be marked as
read by the server too so that they will go through the expiry process?

expiry-wait is set to 30, total-expiry is on too and I use Gnus pulled
just some minutes ago.

Thanks in advance!

-- 
Best regards,
Gábor