I don't know if this is the case with techdirt, but by far the most common reason why sites and domains (for mail) end up getting flagged is because their resources have been hijacked by organized criminals and are being used for crime, unbeknownst to the people who own the domains. So if this ever happens to you your first instinct should be to question your own production security situation, not to assume that Goog…
Playing the guessing game is not helpful. In my experience, things just go dark and you get very little or no helpful messaging about resolving the problem.