private CrmNumber GetNextAttachmentNo(ICrmService service, Guid EmailID)
{
    CrmNumber retval = new CrmNumber(0);
    // condition
    ConditionExpression condition = new ConditionExpression();
    condition.AttributeName = "activityid";
    condition.Operator = ConditionOperator.Equal;
    condition.Values = new object[] { EmailID.ToString() };
    //filter
    FilterExpression filter = new FilterExpression();
    filter.FilterOperator = LogicalOperator.And;
    filter.Conditions.Add(condition);
    // columns
    ColumnSet cols = new ColumnSet();
    cols.AddColumn("attachmentnumber");
    // new query
    QueryExpression query = new QueryExpression();
    query.EntityName = EntityName.activitymimeattachment.ToString();
    query.ColumnSet = cols;
    query.Criteria = filter;
    // request object
    RetrieveMultipleRequest retrieve = new RetrieveMultipleRequest();
    retrieve.Query = query;
    // execute
    RetrieveMultipleResponse retrieved = (RetrieveMultipleResponse)service.Execute(retrieve);
    //Find max attachment number
    foreach (object o in retrieved.BusinessEntityCollection.BusinessEntities)
    {   // cast
        activitymimeattachment eat = (activitymimeattachment)o;
        if (eat.attachmentnumber.Value > retval.Value)
        {   // higher, remember it!
            retval.Value = eat.attachmentnumber.Value;
        }
    }
    // done; we want the next one
    retval.Value += 1;
    return retval;
}