You've done everything right. SPF is configured. DKIM is signing. DMARC is set to reject. Your content has no spam triggers. You're sending to people who opted in. And yet — Gmail puts you in spam.
Before you rewrite your email for the fourth time, check something most guides don't mention: your IP address's neighborhood.
If all of those are checked and you're still hitting spam, the problem is likely upstream of your content. It's the IP.
Every IP address belongs to a /24 block — a group of 256 consecutive addresses. Your IP is one of 254 usable addresses in that block. The other 253 are your "neighbors."
When Gmail evaluates a new or low-reputation IP, it doesn't just look at that IP in isolation. It looks at what it knows about the block. If 5 of your neighbors are on Spamhaus SBL (the most impactful email blacklist), Gmail's prior on your IP is negative — before you've sent a single email.
This is called collateral damage, and it's the most common invisible cause of deliverability problems that authentication alone can't fix.
1. You deployed an app on a VPS. You got IP 45.33.32.100 from Linode. The previous tenant on .98 was running a spam operation. They're gone, but the Spamhaus SBL listing on .98 is still active. Your password reset emails from .100 go to spam because Gmail sees the SBL listing in your /24.
2. You got a dedicated sending IP from your ESP. SendGrid assigned you 167.89.139.42. You start warmup. It's painfully slow — 3 weeks in and inbox rates are still under 50%. You scan the /24 and find it's a ghost block: zero other senders, zero PTR records, zero reputation. Gmail has no context for your IP. You're warming up in a vacuum.
3. You're running a SaaS and transactional emails are failing. Signup confirmations, invoice receipts, password resets — all intermittently going to spam. Your app is on Railway/Render/Fly. The IP you got is in a block with 20 other apps, 3 of which are on blacklists. You never chose this IP. You didn't even know what block you're in.
Enter your IP address in the scanner below. We scan all 254 IPs in your /24 block against 10 major blacklists, identify every active sender, check adjacent blocks for collateral risk, and tell you whether your neighborhood is helping or hurting you.
If the scan shows SBL listings in your block, you've found your problem. The fix depends on your situation:
On a VPS: Ask your provider for a different IP. Most will reassign for free. Scan the new one before accepting it.
On an ESP (SendGrid, Mailgun, etc.): Request a new dedicated IP. Scan its /24 before starting warmup. If it's a ghost block, release and request again.
On a PaaS (Railway, Render, Fly): You usually can't choose your IP. But you can know about it — and if it's dirty, move to a provider that assigns from cleaner ranges, or route transactional email through an ESP with a dedicated IP.
See your /24 neighborhood in 60 seconds. Free, no signup.