You hit send, and your message quietly lands in the recipient's spam folder — or never arrives at all. It's one of the most frustrating problems in email, partly because there's rarely a single cause. Below are the ten reasons legitimate mail gets filtered, roughly in the order worth checking, each with a concrete fix.

1. Missing or broken email authentication

This is the number-one cause today. If your domain has no SPF, DKIM, or DMARC records — or they're misconfigured — Gmail, Yahoo, and Microsoft increasingly treat your mail as suspicious by default. Since 2024, authentication is effectively mandatory.

Fix: Publish all three records and make sure they pass and align. Start with our complete guide to SPF, DKIM & DMARC, then verify with a full domain check.

2. Your sending IP or domain has a bad reputation

Mailbox providers score every sender on reputation built up over time. A new domain, a shared IP that someone else abused, or a recent spike in complaints all drag your score down.

Fix: Warm up new domains gradually, keep volume steady, and check whether your sending IP is listed on a blocklist with our blacklist check.

3. No reverse DNS (PTR) on your mail server

Receiving servers expect the IP that connects to them to have a matching PTR record — reverse DNS that maps the IP back to a hostname. A missing or generic PTR (like a raw ISP-assigned name) is a classic spam signal.

Fix: If you run your own mail server, ask your host to set a proper PTR. If you use a hosted provider, this is handled for you. Our MX lookup shows the reverse DNS for each of your mail servers.

4. Spam-trigger content and formatting

Filters still read the message itself. All-caps subject lines, "FREE!!!", lots of exclamation marks, a single giant image with almost no text, link shorteners, and misleading subjects all raise your spam score.

Fix: Write like a human. Balance text and images, avoid spammy phrasing, and make the subject match the content.

5. Broken or mismatched links

Links whose visible text doesn't match their destination, links to flagged domains, or naked IP-address URLs all look like phishing. So does a "from" domain that differs from the domains you link to.

Fix: Use clean, consistent links on domains you control, and avoid public URL shorteners in bulk mail.

6. No easy unsubscribe (for bulk mail)

For marketing and bulk email, Gmail and Yahoo now require a working one-click unsubscribe and a complaint rate kept below 0.3%. Hard-to-unsubscribe mail gets reported as spam, which poisons your reputation fast.

Fix: Add a List-Unsubscribe header with one-click support, and honour opt-outs immediately.

7. Sending from a free address on a custom domain

Sending "from" you@yourcompany.com through Gmail's servers without authorising them in SPF and DKIM produces a mismatch that filters dislike — and that will outright fail DMARC.

Fix: Authorise every service that sends on your behalf in SPF, and enable DKIM signing for each.

8. Sudden spikes in volume

Going from 50 emails a day to 5,000 overnight looks exactly like a compromised account. Providers throttle or filter to protect their users.

Fix: Ramp volume up gradually over days or weeks, especially on a new domain or IP.

9. Recipients aren't engaging

If people never open, reply to, or move your mail out of spam, providers learn that your mail is unwanted and filter more aggressively over time.

Fix: Clean your list of dead addresses, stop mailing people who never engage, and send content recipients actually asked for.

10. SPF over the 10-lookup limit (silent failures)

SPF allows a maximum of 10 DNS lookups. Add too many include: services and SPF returns a permerror and effectively stops working — often without any obvious symptom.

Fix: Audit your SPF record, remove unused includes, and flatten where possible. See our SPF guide and run the SPF checker to count your lookups.