Most founders think spam filtering is a black box — some mysterious algorithm that decides, seemingly at random, whether your emails land in the inbox or disappear into the void.
It isn't. Spam filters are rule-based scoring systems, and once you understand what they're scoring, you can reliably engineer your way into the inbox.
Here are the seven signals that matter most — and exactly what to do about each one.
1. Sender reputation (the biggest one)
Every sending domain and IP address has a reputation score maintained by inbox providers. When you send from a brand-new domain — even with perfect copy — you start with zero reputation. Gmail and Outlook treat new senders as threats by default.
The fix: Warm up every inbox before sending cold campaigns. This means gradually increasing your send volume over 30 days while generating positive engagement signals (opens, replies, moves out of spam). A reputation score of 70+ is when you can safely start sending campaigns.
2. SPF, DKIM, and DMARC authentication
These three DNS records tell receiving mail servers that you are who you say you are. Without them, even legitimate emails look suspicious. DMARC failure alone can cause 30–40% of your emails to be blocked or quarantined.
The fix: Add all three records to your DNS. It takes 15 minutes and is the highest-leverage thing you can do before sending a single cold email. We cover the exact setup in our SPF/DKIM/DMARC guide.
3. Engagement history
Gmail tracks how recipients interact with emails from your domain. If your last 1,000 emails had a 0.1% open rate and 20 spam complaints, your next campaign starts with a major disadvantage.
The fix: Only email people who are likely to engage. A smaller, targeted list with a 10% reply rate builds your reputation faster than blasting 10,000 cold contacts.
4. Email content signals
Certain patterns in email content trigger spam filters automatically. These include:
- Too many links (more than 2–3 in a cold email)
- HTML-heavy formatting with large images
- Spam trigger words: "free," "guaranteed," "act now," "no obligation"
- All-caps text or excessive exclamation marks
- URL shorteners (bit.ly, etc.)
The fix: Write cold emails like a real person would write them. Plain text, one clear CTA, minimal links. The best-performing cold emails look like they were typed in Gmail, not built in an email tool.
5. Send volume spikes
Sending 10 emails one day and 500 the next is a classic spam signal. Inbox providers expect consistent, gradual volume — the pattern of a legitimate business communicating with real customers.
The fix: Set a daily send limit and stick to it. Ramp up gradually — 20/day in week one, 40/day in week two, and so on. Inboxrise enforces this automatically with configurable send windows.
6. List hygiene
Sending to invalid email addresses (bounces) at a rate above 2% is a major red flag. Hard bounces signal that you're not maintaining your list — a behaviour associated with spammers.
The fix: Verify your email list before importing it. Tools like NeverBounce or ZeroBounce can clean a list in minutes. Inboxrise automatically suppresses bounced contacts after the first failure.
7. Unsubscribe compliance
CAN-SPAM requires a working unsubscribe mechanism in every commercial email. Beyond legal compliance, having a clear unsubscribe option actually improves deliverability — it gives unhappy recipients an alternative to clicking "Mark as spam."
The fix: Include an unsubscribe link in every cold email. Inboxrise injects one automatically. Make it easy to find — not buried in 6pt grey text.
Putting it all together
The founders who consistently land in the primary inbox aren't doing anything exotic. They're doing the basics right: warmed inboxes, authenticated domains, targeted lists, plain-text emails, and gradual send volumes.
Fix all seven of these and your deliverability problems will disappear — not because you've gamed the system, but because you're sending email the way Gmail and Outlook want you to.
The counterintuitive truth: The best way to improve deliverability is to send fewer, better emails to people who actually want to hear from you. Reputation is earned, not engineered.