Just one more reason to keep your lists very, very clean…
According to this article from DIRECT Magazine, MSN maintains "honey pot" email addresses. These are dormant accounts that they’ve created, and are never used to sign up for marketing. They might post them online somewhere, and wait to see if a "bot" ever comes by to grab them. If you send email to one of these "trap" addresses, you’ve obviously been scraping email addresses, or purchased a list from someone who has.
This is why it’s so dangerous to buy email lists from people. It’s also why we highly recommend using double opt-in for your in-house lists (a feature built into MailChimp).
MSN is also taking old, expired, or abandoned email addresses on their system, and turning those into traps as well. The idea is that if you send email to one of their "abandoned" accounts, it means you’re not cleaning your list as often as you should, so you’re probably an irresponsible marketer.
This is one reason (here’s another) why you should try to send to your list on a fairly regular basis. You should also use a list management tool that cleans bouncebacks for you (pretty basic feature built into MailChimp, and most other reputable email marketing services).
The article also provides some tips on how to handle email lists that have been dormant for a while.