"I hope this letter finds you well" is a traditional, fairly formal, possibly slightly old-fashioned greeting. I see no reason why "email" should not be substituted for "letter", except to the extent that emails tend to be less formal, and less likely to be couched in traditional letter-writing phrasing. The author of the article actually says that she thinks "I hope this email finds you" is awkward, but I don't understand whether she is specifically objecting to the use of "email" rather than "letter", or just isn't aware of the expression at all.
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