This is the way with email spam anyways. Not sure if it works the same way with forum spam. With email spam, oftentimes each email has a slightly modified link all redirected to the same inviisible 1x1 image. That way they can know exactly which of their thousand spam Viagra emails has made it to human eyes.
A common technique for non-spam too, such as your motorbike club newsletter that you've signed up for and actually read sometimes - that tells them who has and hasn't read it. They can then play games like
- send to 10% of the membership with headline A
- send to 10% of the membership with headline B
- wait three days
- see which headline prompted more people to read the email
- send the remaining 80% with the more effective headline.
All of which comes with the mailing list software ...