ISP's view too many emails sent together as possible SPAM. That is obviously the policy of Orange and can't simply change it to suit individuals. I don't know about other ISP's but they may have similar policies.
The only way open to you is to send them in batches of 20 (or less) and my advice would be for you to do that as I don't think you have an alternative. I think there may be a way around it if you are prepared to pay but I am unsure about that. Maybe someone else will know more about this than I do.