"The vulnerability allows remote HTML content to be loaded in an email, which replaces the content of the email message. Soucek wrote he then built a functional password collector using HTML and CSS. He also published a demonstration video."
Oh, Good Grief. . . Souçek really is stretching on this one. . . he's the same guy who claims Apple hasn't fixed the other so-called iCloud vulnerability he claims to have found. This time he is claiming the ability of Apple Mail to display HTML and execute HTML scripting is a vulnerability because it can be used for phishing attacks against users of email.
The standard rules of using email of not inputing any user names or passwords into any email or attachments still stand and clicking on any links in an email is a stupid thing to do, regardless if it is a imbedded HTML or a remote HTML installed by a script in the email. Whichever it is, it is still a PHISHING attack on the user, nothing more.
Almost all email clients can display HTML and it is not a vulnerability and is an intended feature. It can be turned off if the user does not wish to see HTML displayed content. In fact, email containing HTML can be blocked from being received.

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