Constant Contact, MailChimp, etc, all work the same way. They have to err on the side of protecting everyone from spam or risk having their entire platform, and everyone using it, blacklisted.

It's interesting that you just posted this, as @StellaBelle just wrote a post about the #WarOnSpam. You can check it out here.

The war on spam is everywhere and affects us all. Sometimes in ways that feel unfair.