Please do not use NiXSpam or encourage others to use it.
If I remember correctly they are pretty trigger happy with honeypot email addresses that are pretty easy to find on the internet.
All your competition needs to do is register to your service using the honeypot email addresses and your legitimate email server/domain will be blacklisted on hosts that check NIXSpam.
* don't accept mail for non-existing recipient adresses
* don't accept mail where the sender domain is one of my domains, except when whitelisted manually
* don't accept mail from hosts on the NiXSpam list[1] (unlike some others, NixSpam has pretty reasonable policies with automatic delisting after 12 hours)
These three rules together work pretty well, I get only a handful false negatives every day, and virtually no false positives, even though I publish my real mail address on many places in the web, and even used it in usenet, back in the days.
Next step would be using SpamAssassin, but [2].
[1] http://www.heise.de/ix/NiX-Spam-DNSBL-and-blacklist-for-down...
[2] http://xkcd.com/1205/