Most people treat spam filters like a black box. You send an email, and it either lands in the inbox, the promotions tab, or the dreaded spam folder. It feels random. But it isn’t.
Spam filters don’t guess. They run your emails through a grading system, scoring everything from your technical setup to your sending habits to your audience’s engagement. One weak link and you’ll find yourself filtered out.
This guide breaks down exactly how spam filters grade your emails, what factors matter most for email deliverability, and how you can align your strategy to win.
1. Authentication Check: Are You Who You Say You Are?
Before content, before reputation, filters look at authentication.
- SPF (Sender Policy Framework): Verifies which servers are allowed to send on your domain’s behalf.
- DKIM (DomainKeys Identified Mail): Signs your messages digitally to prove they weren’t tampered with.
- DMARC (Domain-based Message Authentication, Reporting & Conformance): Tells providers what to do if SPF or DKIM checks fail.
Fail these checks and you’re waving a red flag to Gmail, Microsoft, and others. It’s the equivalent of showing up without ID; you won’t get through the door.
Best practice: Configure all three records for every sending domain. Use tools like MXToolbox or integrated email deliverability services (like Mission Inbox) to check if your DNS is airtight.
2. Reputation Score: Your Sending History Matters
Spam filters track your domain reputation and IP reputation over time. Think of it like a credit score: good behavior earns trust, bad behavior makes providers cautious.
High bounce rates = poor reputation.
- Spam complaints = major penalties.
- Blacklist appearances = severe trust loss.
-
- Shared IPs in traditional SMTP relay services or bulk email platforms can also drag you down if other senders misuse them. This is why many outbound teams switch to deliverability-first platforms with domain rotation and monitoring.
-
- Best practice:
- Age new domains before sending.
- Warm them gradually.
- Monitor reputation daily.
3. Content Scan: What’s Inside the Email
Filters also scan the content itself. They don’t just look for “FREE!!!” in the subject line anymore; they analyze patterns.
- Keywords: Excessive promotional language, deceptive subject lines.
- Formatting: ALL CAPS, too many images, missing text-to-image ratio.
- Links: Multiple redirects, URL shorteners, or mismatched domains.
- Attachments: Especially .exe or other risky files.
Even if your infrastructure is flawless, sloppy copy or excessive links can still push you into spam.
Best practice:
- Write natural, value-driven copy.
- Use clean formatting (HTML + plain text).
- Limit links, and avoid URL shorteners.
-
4. Engagement Signals: How People React to You
Mailbox providers weigh engagement heavily. Why? Because they want to give users what they interact with most.
Positive signals: Opens, clicks, replies, moving emails from spam to inbox.
- Negative signals: Deletions without reading, spam reports, ignoring your emails consistently.
-
- A high percentage of disengaged recipients tells Gmail or Outlook: “this sender isn’t relevant.” That tanks your inbox placement across the board.
-
- Best practice:
- Clean your list regularly.
- Segment and personalize.
- Focus on relevance > volume.
-
- This is where many bulk senders fall down. Bulk email sending services make it easy to blast thousands of contacts, but without engagement, you burn your reputation quickly.
-
5. Final Verdict: Inbox, Promotions, or Spam
After checking authentication, reputation, content, and engagement, spam filters make the final call. And this verdict isn’t fixed; it adapts in real time.
Two senders can send the exact same email, but one lands in inbox and the other in spam. Why? Because the scoring system is different for each domain, IP, and history. That’s why ongoing monitoring is critical. Without testing and data, you’re guessing blind.
Why Spam Filters Are Stricter in 2025
Filters are harsher than ever. AI-driven models now analyze context, intent, and patterns across billions of emails daily. This means:
- You can’t rely on tricks (like spinning subject lines).
- You can’t brute-force with volume.
- You can’t assume that one-time fixes will hold.
If you’re serious about outbound or transactional email, deliverability discipline is the only long-term solution.
How to Align With the Grading System
Here’s what winning teams do differently:
- Nail the technical setup. Every domain authenticated, tested, and monitored.
- Warm up domains gradually. Never send cold from a fresh setup.
- Separate traffic types. Keep transactional and marketing on different domains/IPs.
- Monitor constantly. Blacklists, spam placement, reputation scores.
- Engage smartly. Segment, personalize, and measure replies — not just opens.
The Role of Deliverability Services
Manually checking every factor above isn’t sustainable. That’s where modern email deliverability services step in.
Tools like Mission Inbox combine:
- Spam filter testing (see where your emails actually land).
- Mailbox/domain warmup to build reputation safely.
- Blacklist monitoring to detect and fix issues early.
- SMTP relay service integrations to manage scaling.
This combination gives you visibility and control — and keeps you aligned with how filters grade your emails.
Final Takeaways
Spam filters don’t hate you. They just grade you.
- Fail authentication? You lose points.
- Burn reputation? You lose trust.
- Ignore engagement? You look irrelevant.
Pass the grading system and your emails make it to the inbox. Ignore it, and you’ll spend more time in spam than in front of prospects.
If you’re relying on guesswork, you’re already behind.
Want to see how spam filters are grading your emails right now? Run a deliverability test with Mission Inbox.