Email Encryption

Email generally is not encrypted unless you use a common encryption format like PGP.

https://gpgtools.org/

Is a utility that lets you use PGP on your Mac. If you want to save an email into an encrypted file, you can with your email program save the message as plain text onto your desktop and use Disk Utility to create an encrypted image to put the file in.

As long as the recipient has the password for that image, it will be secure.

https://www.ed.ac.uk/infosec/how-to-protect/encrypting/encrypted-containers/mac-create-disk-image

You don't need any 3rd party software to send or receive encrypted e-mails. Apple Support even has a support documented showing how to set this up. Read Sign or encrypt emails in Mail on Mac - Apple Support

The hardest part is getting and setting up your personal encryption certificate. Apple has a support page for this too. See Use personal certificates in Mail on Mac - Apple Support

In most cases, you will be expected to pay for such a certificate. I found this site claiming to offer a free one. I don't use this service and I'm not recommending it. You might be able to find a free certificate elsewhere. I don't know.