I didn’t get any spam either, but then a short grep through my mail.log showed this:

  2013-02-28T18:05:18.865406+01:00 nfc postfix/smtpd[14995]: NOQUEUE: reject: RCPT from bl14-172-78.dsl.telepac.pt[85.247.172.78]: 504 5.5.2 <discus>: Helo command rejected: need fully-qualified hostname; from=<fuzzilyjg755@lanuschny.de> to=<X_dropbox@example.net> proto=ESMTP helo=<discus>

It is as if spammers don't even try anymore! Bogus helo is soon 1990s..

Postfix’s reject_invalid_helo_hostname, reject_non_fqdn_helo_hostname, and reject_unknown_helo_hostname really work wonders for me – but then it is only a small server with 1.5 users and a total of about 300 delivery attempts a day (85% rejected, 5% later classified as spam, 10% actual email).