They worried all the time about it; systems weren’t perfect.

Interception was usually the bigger issue. Messengers would have orders to destroy their messages if caught, but that didn’t always happen. You might even have just a failure to transmit a message; a lot of the theory for modern Internet packet switching came from military communication.

Forgery could happen. This was usually counteracted by knowing the sender’s handwriting or applying a seal or stamp to the communication. It wasn’t perfect, but it did ok.