How to search mails from custom folders via EV Search API?

I'm using EV search api to search emails. However, I can only search the mails from system folders such as "Inbox", "Sent Items", etc. Our customer created custom folders such as inbox/firm, inbox/helpdesk. I cannot search the mails under those custom folders. Any idea?

This is how I get my achives:

            IArchives archivesRead = (IArchives)cmAPI.Archives;
            archivesRead.Computer = "192.168.24.26";
            archivesRead.Permissions = EV_STG_API_PERMISSIONS_TYPE.PERMISSIONS_SEARCH;
            archivesRead.ArchiveTypes = (int)EV_STG_API_ARCHIVE_TYPE.ARCHIVE_TYPE_MAILBOX;
            archivesRead.Get();

This is how I use the search api:

                ISearchQuery2 query = (ISearchQuery2)new SearchQuery();
                IIndexSearch2 search = (IIndexSearch2)new IndexSearch2();

                search.SelectArchive(archiveId);
                query.AddTerm("rtea", "xx@dd.com", 0);
                //query.AddOp((int)ESearchQueryOperators.ESQand, (int)ESearchOperatorScope.ESQdefault);
                ISearchResults2 results = (ISearchResults2)search.Search(query.Query, 1, 10000, "");