There's rarely one magic reason. Mailbox providers add up lots of small signals, and when enough of them look bad, your email goes to spam. Start at the top of this list: the first few causes are the most common and the quickest to check.
1. SPF, DKIM or DMARC is missing
These three DNS records prove the email really comes from you. Without them, Gmail and Yahoo treat bulk mail as suspicious, and since 2024 they reject a lot of it. Check your domain; it takes ten seconds and tells you exactly what to add.
2. The domain or IP is on a blacklist
One listing on Spamhaus or SpamCop can quietly hurt everything you send. Run a blacklist check. If you're listed, fix the cause first, then ask for removal.
3. The domain is brand new
A domain registered last week has no reputation. Big sends from it land in spam almost every time. Warm it up for two to four weeks first.
4. You're sending too much, too fast
Hundreds of emails in a few minutes from one account looks automated. Spread sending across accounts and add pauses. See the limits for Gmail, Outlook and Yahoo.
5. Your list has old or bad addresses
Lots of bounces tell providers you don't know your recipients. Remove addresses that bounced and people who haven't opened anything in months.
6. People mark you as spam
This is the strongest signal of all. Gmail wants complaints under 0.3%. Only email people who expect it, and make unsubscribing easy. A visible unsubscribe link is better than a spam click.
7. The text looks like spam
ALL CAPS, "FREE!!!", "act now", piles of links and link shorteners all add points. Paste your email into the spam word checker and fix what it highlights.
8. Every email is identical
Hundreds of copies of exactly the same message are easy to fingerprint. Use the person's name, and vary the subject or the first line.
9. Nobody replies
Replies and "not spam" clicks are the best signals you can get. Ask a real question people want to answer. For new inboxes, a warmup service creates that early activity for you.
A quick order of attack
- Fix DNS (SPF, DKIM, DMARC).
- Check blacklists.
- Slow down and spread the volume.
- Clean the list.
- Rewrite the email so it reads like a person wrote it.