For those of you who don't know what a spam trap is, here are the basics:
A spam trap (or a honey pot address) is an email address that is created for the sole purpose of catching senders of unsolicited email. Any email sent to these addresses is by definition spam. And if you're caught doing so you will immediately be put on the ISP's blacklist.
How do spam traps work? ISPs typically post honey pot email addresses on different websites waiting for them to be "scraped" or "harvested" by spammers or not-so-honest list brokers.
How to avoid emailing honey pot addresses? You should be very careful mailing to lists of which you are not sure how permission was obtained (eg. rented or bought lists, lists from marketing partners...). Make sure you always know if and how they obtained permission for these addresses before using these lists.
It gets more complicated now that Microsoft has decided to turn abandoned Hotmail accounts into spam traps. Microsoft doesn't say how long the account must be abandoned before they turn it into a spam trap, but they are a bit more forgiving when companies send email to abandoned accounts than they are for companies emailing a honey pot address.
The best way to avoid emailing abandoned Hotmail accounts is list hygiene. Make sure to remove bounced addresses from your mailinglists, be careful with sending email campaigns to a list that you haven't used for more than six months and be extra careful when using 3rd party email lists.
What to do if spam traps appear on your list? Joshua Baer, chief executive of email service provider Skylist, recommends this: try to isolate the problem by halving the list and testing mailings to it until the source of the bad data becomes apparent. How? By dividing the list by source (the website they signed up at, the partner that supplied the email address, the date the addresses were acquired, who signed up before the problem appeared and who signed up afterwards...). Once you've found the culprit, take the necessary action to avoid this from happening again.
Tip: Microsoft offer emailer campaign data (including how many spam traps they're hitting) at http://postmaster.msn.com.