Yes, sending any part of a plant covered under CITES including seed would be illegal. Good thing the CITES list is relatively small. Most orchid seed would be fine.
So I guess then it more a matter of if the reciever minds if you have a phyto or not.
Different countries have different regulations that may or may not require a phytosanitary certificate. CITES is in international treaty, so if it's on the list shipping it gets sticky. If it's not a CITES plant, only individual countries' laws apply. You would have to check your own location to see what is required.