Once again, centralization begets centralization.  I can't tell why GMail is spam-blocking my email, but I'm almost certain it wouldn't if I was sending it through GMail itself.

I endorse the observation that centralization begets centralization, but this data point is false -- your "almost certainty" is wrong. I know of at least one gmail using organization that you're very familiar with, and IIRC 2, that people have been complaining about gmail marking their mail as spam very recently.

Not exactly the same thing, but there's also http://gondwanaland.com/mlog/2013/09/16/googlegroups-found-spam/

The nefarious conclusion might be the same in both cases though: email is hard even for Google, G+ is "better", all ought use that for all their communications. Now there's some centralization. :-/