Email service providers may use complicated anti-spam filters. These filters generally determine which email is SPAM based on their experience and practices. Some of the most common reasons for emails to end up as SPAM are:
- The originating domain name – your domain – is on a blacklist and mail server IP address is blacklisted.
- The receiver email host might require additional validation, such as the existence of SPF(Sender Policy Framework), or DKIM(DomainKeys Identified Mail) for your domain. DKIM record is used to validate the authenticity of email messages. SPF record is to prevent spammers from sending messages with forged From addresses at your domain.
- Your local IP address, assigned to you by your ISP(Internet Service Provider), might be on a blacklist as well.
- Custom anti-spam filters applied by the receiver email host might also end up to dismiss an email based on its contents (subject, body, attachments, etc.)