Why my Query is not fetching the right results

[MishiR]

Hi, am trying to retrieve un-responded messages from the extracted data from Apache project. I thought the following query will work but somehow I still end up with replied messages in the result. For example, am using the following archive: http://mail-archives.apache.org/mod_mbox/httpd-dev/199503.mbox/browser
and am running the following query:
select m.*
from messages m
where m.is_response_of is Null -- original message
and m.message_id not in (select mi.is_response_of from messages mi where mi.is_response_of is not Null) -- not replied
and m.subject not like 'Re: %'