From a recent email I gather this is very off-topic, so I will try to be brief in my reply.

(Geneva.CH.EU.*) since 3+ years. I can say from my experiences I couldn't make any kind of communication between botnets and spam. Most Trojan codes I have looked into doesn't have any command/action to make users send spam emails and such however they try infecting others by different ways that port 25 can't avoid.

Short answer: Many of the infected systems have more Trojan horses installed than the original infecting agent. Many Trojan horses have spam-sending or relaying capabilities.. or even just anonymity capabilities or ID theft/DDoS options. Thing is, there are always more options that can be later installed. As to you not seeing them.. I suppose it's as simple as you not seeing enough of them.. count your blessings. Gadi.