On November 19, 2002, I was getting 10-20 TCP connections per second from around 300 different IP networks to my machine at a colocation facility. I checked the source IPs and they were coming from all over the globe. The destination email addresses all conformed to a simple pattern; this indicated that something was performing a simple algorithmic attack. My computer was really sluggish from queuing all of the bounce messages. My qmail queue was over 13,500 messages at that point. In fact, I couldn't even send out email through the machine's localhost interface. The load caused all sorts of timeouts for other systems on the machine.
My worst spam problem was being the recipient of a joe job. I received around 200 bounce messages to one of my email accounts. After doing some searching on Google, I learned this is a very common problem.
I despise spammers.