1. Is your domain or server on a blocklist?
A listing on Spamhaus or another major blocklist can send everything to spam, or get it rejected. Run your domain through the blacklist checker; it also checks your mail servers' IP addresses. If you're listed, follow how to get removed from Spamhaus.
2. Do SPF, DKIM and DMARC pass?
Without these, Gmail and Outlook can't tell your mail apart from someone pretending to be you. Check them with the SPF & DMARC check and the DKIM checker.
A quick real-world test: send yourself an email in Gmail, open it, choose Show original, and look for PASS next to SPF, DKIM and DMARC. Each service that sends as your domain (newsletter tool, helpdesk, invoicing) needs to pass too. The full setup is in SPF, DKIM and DMARC explained.
3. Did your sending volume jump?
A new domain, or one that suddenly sends far more than usual, looks like a spammer. Raise volume gradually; the warmup planner gives you a day-by-day schedule, and this guide explains why.
4. Are people engaging, or complaining?
- Spam complaints above 0.3% at Gmail cause real problems; aim for under 0.1%.
- Bounce rates above about 2% mean an old or bought list. Remove addresses that bounce.
- Stop emailing people who haven't opened anything in months; they drag down your reputation.
- Make unsubscribing easy. If it's hard, people press "Report spam" instead.
5. Is the mail server set up properly?
If you run your own mail server, make sure its IP address has reverse DNS that matches its hostname, it uses TLS, and it isn't an open relay. The SMTP test and port check show how your server answers.
6. Then look at the content
- Link shorteners and links to domains with a bad reputation can trigger filters.
- A single large image with little text looks like spam.
- Mismatched sender names, all-caps subjects and too many exclamation marks don't help.
Content matters less than most people think, though. Fix steps 1–4 first.