Well, hotmail is telling you that they've received complaints about emails coming from the IP address of your server.
If *you* didn't send those emails, and you're on a shared-server hosting account, then *you* probably will have no voice when talking to hotmail.
If it's a shared server, then your hosting company will have to negotiate with hotmail.
Sounds like your host is lazy and doesn't care.

Whatever the case, this really has nothing to do with Zen Cart ...