- No authentication (missing SPF, DKIM, DMARC)
- Buying or scraping email lists
- High complaint rates (people marking you as spam)
- Overuse of spammy words (free, urgent, limited time)
- Poor text-to-image ratio
- Sending from a shared or blacklisted IP
- Neglecting inactive subscribers
Fixes
- Set up authentication correctly.
- Only send to people who opted in.
- Segment your list and remove inactive contacts.
- Test before sending with inbox preview tools.