US Law: does Mail or Wire Fraud require that the scheme has a target that is per se illegal?

Clearly, this offense (and the similar offense of Wire Fraud) envision acts that could be in preparation for or consitute an attempt to commit an offense that would have been illegal under other laws. For example, mailing a forged check with the applicable mental elements could be considered False Pretenses as well as Uttering a Forged Document (or an attempt to commit them e.g. if the victim catches the forgery). The Mail Fraud statute seems to say that in addition to these basic offenses, the fact that the mail was used results in an additional offense being committed that is separate and distinct.

My question is whether or not it is essential that the end goal, the “scheme or artifice to defraud, or for obtaining money or property by means of false or fraudulent pretenses, representations, or promises” must itself be a goal that would still have been illegal had it not been for the Mail Fraud and/or Wire Fraud statute. For example, if John wants to engage in an underhanded, dishonest, and uncharitable business venture using the US Mail system and/or interstate wire communications systems, but he knows or believes that the scheme is legal because in the landmark case People v. McKendrick, the court reluctantly ruled that the behavior was technically outside the statutory and common law definition of “Fraud” because it was naughty in a very novel way, but the scheme is clearly underhanded and John’s mother disapproves strongly of it, does that prevent him from forming the Intent to Defraud that is an element of the offense?

What would result if John sincerely but mistakenly believes that his hard-sell cold-call selective-disclosure land in Florida real estate sales plan is legal? He might end up committing the end-result offense of Fraud, Real-Estate Fraud, Practicing Realty Without a License, or something else, but is it a defense to a charge of Wire Fraud or Mail Fraud that he is plan was not a “scheme or artifice to defraud” because he did not intend to defraud because he did not know that what he was doing constituted defrauding someone?

In a sense, is it possible in a common-law country to be considered by the law to have actually and legally “defrauded” someone but go unpunished because the specific act, although it was fraudulent, was legal, or does the verb “defraud” necessarily involve criminal conduct and any legal act committed with a non-criminal mental state/mens rea cannot possibly be considered fraudulent in a court of law?