Most email service providers use complex anti-spam filters. These filters usually determine which email is SPAM based on their experience and practices. Some of the most common reason for emails to end up as SPAM are:
- The originating domain name - your domain - is on a blacklist.
- The mail server's IP address is blacklisted.
- Your local IP address, assigned to you by your Internet Service Provider, might be on a blacklist as well.
- The recipient mail host might require additional validation, such as the existence of SPF or DKIM for your domain.
- Custom anti-spam filters applied by the recipient email host might also end up rejecting an email based on its contents (subject, body, attachments, etc.).