It's mostly because you're not yet proved to be NOT spam. When you start a new mail server on an IP there's zero diference between a legimate email server and a

It's mostly because you're not yet proved to be NOT spam. When you start a new mail server on an IP there's zero diference between a legimate email server and a spamhost. Your IP will not be in spamhaus and other blacklists, but you might get stuck in McAffee and other more enterpricey filters, especially if some other IPs in your subnet are known to send spam.