Message Pane visibility should be remembered per folder, as in Outlook. And should not be even visible for Spam, Junk, Virus and similar folders by default.
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I am curious. Why should it not be shown?
There is no security reason that I can establish for not showing it and not seeing the content of the message makes determining false positive spam mails much harder to determine.
Most spam is a fraud or malware. Unless you are a spammer you don't want to click links in spam or open files. Not to say of possibility of some vulnerability.