For what it's worth.
A short while ago I received spam from......myself!!! On one free mail-account I received spam from that same mailadress. I watched it for three days and surely got about 4 mails from myself a day with interesting offers for Viagra, casino's, watches etc. I was, without knowing it, part of a spambot network.
I cleaned my system using a number of free anti-spyware/addware programs and with PC-Tools Spyware doctor. I also had a couple of anti-virus programs scan my computer (in addition of Avast)
I discontinued the e-mail adress.
I never would have found out if I used the automatic spam-filter.