[tor-relays] Received abuse complaint - how did this happen?

Today my ISP informed me that an abuse complaint had been lodged against me by spamcop.net.

I looked at the report and my IP is indeed in it. It seems like a valid complaint. My question, though, is how did this happen?

I'm using the Reduced Exit Policy as shown here:

https://trac.torproject.org/projects/tor/wiki/TheOnionRouter/ReducedExitPoli...

Neither SMTP nor SMTPS are among the ports accepted as exit ports.

Anyone have any thoughts as to how my Tor config can be used to transmit spam?

Thanks.