Don't take any of this the wrong way - but any of these are possibilities:
- Are you SURE you are not an open relay? Have you tested from external IPs?
- What content of messages are you sending? Are your messages all legitimate to opted in customers/clients/etc.?
- Do you have any compromised accounts? Have you reviewed your maillog for traffic?
- Have you added yourself to the feedback loops of major ISPs to know what mail from your servers people are reporting as spam?
State University of New York at New Paltz