Every ISP has custom anti-spam filters which recognize SPAM by:
1. The originating domain name - your domain might be associated with sending spam after customer complaints.
2. The server's IP address being blacklisted.
3. Custom anti-spam rules which look for certain words in the body / header of the email. For example 'Viagra', 'Free promotion' and similar.
4. Others custom antispam rules which are specific to the ISP.
