1. Bad list quality
Invalid addresses bounce, and bounces damage your reputation fast. Old or bought lists are the usual culprit.
Fix: build fresh lists from live sources, verify emails, and remove duplicates and role addresses that bounce. See verifying emails.
2. Missing authentication
Without SPF, DKIM and DMARC, inbox providers can't confirm you are who you say you are.
Fix: add all three records to your sending domain. See SPF, DKIM and DMARC.
3. Too much, too fast
A new inbox sending hundreds of emails in a day is a classic spam pattern.
Fix: warm up, keep daily volume per inbox modest and spread sends over hours.
4. Spam complaints
When people mark you as spam, providers listen.
Fix: target better, write more relevant emails, make opting out easy and honour it immediately.
5. Content that looks like marketing
Heavy HTML, many links, images, tracking everywhere and aggressive sales wording look like bulk marketing.
Fix: plain text, one link at most, no attachments, natural language.
6. Shared or poor sending infrastructure
Some tools or IPs have poor reputation.
Fix: use reputable sending providers and your own authenticated domains.
7. Everyone gets the same email
Identical emails to thousands of people are easy to spot.
Fix: segment and personalise; vary the first line. See personalising at scale.
How to check
Send test emails to inboxes you control at the major providers and see where they land. Watch bounce and reply rates in your sending tool. If something changes suddenly, stop and investigate before sending more.