The problem with relying on legal formulas here is that you need to rely on many people -- hundreds, maybe even thousands -- to not break the law over a long period of time. Your email is not really "sent," it is copied from system to system, and anyone with access to those systems could potentially read it. Backup tapes may be lost or stolen. Hard drives full of email may be sold off. This is not privacy; it is trusting hundreds of strangers to keep your confidence for an indefinite period of time.
And we can do that with cryptography, with even more sophistication than a simple "yes/no" formula:
https://en.wikipedia.org/wiki/Attribute_based_encryption
The problem with relying on legal formulas here is that you need to rely on many people -- hundreds, maybe even thousands -- to not break the law over a long period of time. Your email is not really "sent," it is copied from system to system, and anyone with access to those systems could potentially read it. Backup tapes may be lost or stolen. Hard drives full of email may be sold off. This is not privacy; it is trusting hundreds of strangers to keep your confidence for an indefinite period of time.