Content-related reasons:

1. Suspected spam subject lines: Avoid capital letters, excessive exclamation marks, or typical spam words like "free" or "guarantee".
2. Poorly formatted content:
   • Emails with large images, little text, or poorly formatted HTML can trigger spam filters.
3. URLs in the email:
   • Links to untrustworthy or unverified websites can be marked as spam.

Technical reasons:

1. Missing SPF/DKIM/DMARC entries:
   • These authentication protocols are important to mark your emails as secure.
2. IP address on blacklist:
   • If the IP of your email provider is on a "blacklist", your emails will likely end up in spam.
3. Shipping via free email services:
   • Business communication via free email services such as GMX, Yahoo, or Gmail can appear unprofessional.

Behavioral reasons:

1. High email volume:
   • Too many emails in a short period of time can be considered as spam.
2. Low interaction rate:
   • If recipients rarely open or respond to your emails, spam filters may be blocking them.
3. Emails are being marked as spam:
   • If many users mark your emails as spam, it will have a negative impact on your sender reputation.

Other reasons:

1. Inconsistent sender information:
   • The "From" name should match the content and purpose of the email.
2. Missing unsubscribe option:
   • A clear unsubscribe link is legally required and necessary.
3. Spam traps:
   • Sending emails to spam trap addresses accidentally can damage your reputation as a sender.

To avoid spam filters:

• Improve your content: Compose professional and clear emails.
• Authenticate your emails: Implement SPF, DKIM, and DMARC for your domain.
• Ask customers for whitelisting: Customers should add your address to their address book or safe sender list.