Here is the sum total of electronic communications, s.
If you absolutely, positively cannot risk the slightest chance of someone intercepting your electronic transmission. Don't send it.
This includes internet, telephone, cell, coded CB, direct satellite or any other electronic means of communication.
If a person has enough resources and/or access to the link, they can get your transmitted data.
The only thing you can do to mitigate the problem is to encrypt all the data you can so that even if they do get it, it will be very difficult for them to decipher what you have sent.
If the person is intent upon spoofing your identity (for whatever reason) they will be able to accomplish this if they are good enough, but that too can be mitigated but is way beyond being explained here on TechPowerUp.
Even if you travel to the person who is to receive the message and tell them verbally, there are ways to listen in to the conversation, even remotely.
If the person watching you has that many resources, you are probably already compromised in many ways.
My advice ... just use a secure transport protocol and make sure both the transmitting and receiving computers are protected with good firewalls to prevent intrusion, and you should be fine.