We only mail people who sign up!
I get a lot of calls from clients who can’t understand why they have spamtraps on their lists. Most of them tell me that they never purchase or rent lists, and they only mail to people who sign up on their website. I believe them, but not all of the data that people input into webforms is correct.
While I don’t have any actual numbers for how many people lie in forms, there was a slashdot poll today that asked readers “How truthful are you when creating web accounts?”. The answer seems to be “not very” at least for the self-selected respondents.

Companies finding themselves with ongoing spamtrap problems when they are only collecting data through their own websites need to take a step back and look at their overall process. Often there are minor changes that can be made to lower the amount of invalid information submitted. Sometimes, though, there needs to be more aggressive data verification as part of the subscription or signup process.
We have helped a number of companies improve their signup processes. Those who implement our suggestions see improved delivery and fewer blocks as well as a more engaged and profitable audience.