To be clear, the problem is not that Apple is flagging terms that are most often used in unwanted, spam messages--it's the lack of transparency about this filtering. Apple is flagging messages that it seems very sure are spam, but it has no real system in place for dealing with false positives--messages that are filtered, but aren't actually spam.
What makes this more confusing for some users is that Apple already has spam handling procedures in place: Both iCloud and Mail on the Mac offer Junk mail folders, but the "barely legal" messages never even appear there. Apple presumably feels so certain that specific messages are spam that it's choosing to bypass the Junk folder entirely. That could be okay, if Apple did more to let users know what was happening.
Other services have different ways of handling problems like this: Many send you reports of messages that were blocked, and allow you to indicate when an email has been mistakenly flagged.
Apple's filter is also completely opaque to users: While we now know that the phrase "barely legal teens" is blocked, we don't know what other phrases might be on Apple's blacklist. And there's no way to find out, short of trying to repeatedly send emails with potentially triggering phrases in them.
And even if you do discover something Apple is filtering, calling up AppleCare and telling them that you didn't receive an email containing "barely legal teens"--or some other potentially crass phrase that you might discover Apple filters for--is not an experience anybody really wants to have.
Moreover, our sources indicate that even if you do contact them, AppleCare staffers can't actually recover the missing emails. So you can report that you're missing an important email, and maybe the AppleCare reps can do something--whitelist the sender, perhaps?--but you won't actually get the missing message unless you tell the sender to try emailing it again.
At best, you might hope that multiple reports of this kind might spur Apple to change its policy, but it seems unlikely that Apple will decide to stop filtering for those phrases, no matter how many people complain about not receiving emails with potentially offensive phrases.
Junk in the trunk
Some might be willing to forgive this practice if Apple's approach had magically solved the spam problem. But spam still occasionally gets through iCloud's many filters--and that's why the service offers a Junk folder. Apple's choice to delete certain messages before they arrive may help in the war against spam, but there's too much risk of collateral damage.
It's also worth noting that there may well be some customers who genuinely wish to receive emails containing precisely the sorts of content Apple's filters seek to ban. While Apple's prudish tendency feels at least defensible on the App Store, it seems puritanical when it comes to pre-screening a customer's private inbox.
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