You should assume that all email services, whether free or paid auto scan your emails. They need to do so for spam/malware detection. It's what they do with the scanned info that is not always clear?
As mentioned, never write anything in an email that you don't want a third party to read. There are technical ways, and services that can secure e-communications. Though these tend to require some type of setup or account creation from both you and the email recipient(s).
So if you want to keep your current email address, and need to send something private? Then you may want to send the message via an strongly encrypted attachment? Contact the recipient via phone, and and give them the password key? Never send the password by another email.
The free 7-zip can create a strong encrypted AES-256 archive attachment easily from any file.