Search appointments by required or optional attendees

Hi there

I need to search appointments by required or optional attendees using EWS. I've tried with a search filter collection and a query string but none of these options worked.

Search Filter Collection

var attendeeFilter = new SearchFilter.SearchFilterCollection(LogicalOperator.Or)
            {
                new SearchFilter.ContainsSubstring(AppointmentSchema.RequiredAttendees, mailAddress),
                new SearchFilter.ContainsSubstring(AppointmentSchema.OptionalAttendees, mailAddress),
            };

FindItemsResults<Item> results = ews.FindItems(WellKnownFolderName.Calendar, attendeeFilter, view);

Query String

string queryString = string.Format("(RequiredAttendees:{0} OR OptionalAttendees:{0})", mailAddress);

FindItemsResults<Item> results = ews.FindItems(WellKnownFolderName.Calendar, queryString, view);

Does anyone know the magic trick to achieve this?