The real problem is Google is opaque in dropping such mail in recipients spam folder. Hotmail can sometimes be annoying in this regard too. As far as I've managed neither does the sane thing and add a header that report on how they've divided that "this mail is spam" - so the recipient doesn't have anything to go on either, other than hoping "mark this as bot spam/add to address book/send email/reply to address" help Google/Ms treat it as "not spam".

Maybe it's worse for non-English mail (maybe the statistical models are biased against "not English" even for people whom don't have English as a native language).

But I'm quite convinced google's (and to a lesser extent Hotmail/ms') "magic" spamfiltering is subtly (but annoyingly) broken.

I'm definitely not giving to somebody else my /var/log/mail.log, IMAP/Mutt access, sieve rules, nor ad hoc dedicated aliases for every website account I create. And on top of that, I would land on some US-based server for ease of illegal spying and my mail would be harvested for some advertising crap, all to solve something that is not a problem for me. Not happening.