How to avoid overflowing network Channels

Hi, I need to send a lot of data from server to clients. I use "NetworkServer.SendToClient". I need to send multiple "packets" because one message can have max. size of 65536B. When I send more than 5-6 at the time, I get channel overflow errors, or data gets partially lost. I avoided it by sending one message every second, but it is slow.

How can I avoid overflows? How could I use other channels and would it help? How can I check on server side, if message sent by "NetworkServer.SendToClient" has arrived to client?

Thanks.