This is because the recipient might have a compromised device. In other words, the device may have a virus, malware, or been compromised by a phishing attack. Even if the recipient has a computer with good virus and malware protection, she could just forward the email to anyone and you'd never know.

Why does my email say not secure?

The reason you are seeing the “Not Secure” warning is because the web page or website you are visiting is not providing an encrypted connection. When your Chrome browser connects to a website it can either use the HTTP (insecure) or HTTPS (secure).

What does send unencrypted mean?

not enciphered or encoded; not encrypted: Unencrypted data sent over the internet can be intercepted by hackers.