TL;DR
Set up SPF/DKIM/DMARC authentication, warm up your sending domain, keep your list clean (under 3% bounce rate), avoid spam trigger words, send plain-text emails, and maintain a low complaint rate by only emailing genuinely relevant prospects.
Spam filters are sophisticated and getting smarter every year. Here's what actually determines whether your email lands in inbox or junk:
Technical infrastructure (hardest to fake, most important): - SPF, DKIM, and DMARC configured correctly on your sending domain - Domain aged and warmed up (new domains sent cold immediately = instant spam) - Sending domain separate from your main company domain - Consistent sending volume (don't send 10 emails today and 500 tomorrow)
List quality: - Keep bounce rate below 3% — validate emails before sending - Only email people who fit your ICP — high complaint rates from irrelevant outreach are a signal - Include an unsubscribe mechanism (required by CAN-SPAM and GDPR) - Remove unsubscribes and bounces from all future sends
Email content: Spam filters scan for: - Links (especially shortened URLs or redirects) — minimize links in cold email - HTML with tracking pixels, large images, or complex formatting — use plain text - Spam trigger words: "Free," "Guarantee," "Click here," "No risk," "Special offer," "Act now," "100%" - Excessive capitalization and exclamation marks - Subject lines that look like marketing templates
Behavioral signals: Gmail and Outlook track engagement. If people open your emails and reply, that's a positive signal. If they mark you as spam or your emails go unopened, that damages your reputation. This is why personalization matters beyond reply rate — it drives engagement signals that keep you in inbox.
Monitoring tools: - Google Postmaster Tools: Free tool to track your domain's reputation with Gmail - Mail-Tester: Send a test email and get a spam score - GlockApps: Tests inbox placement across major providers - Instantly and Smartlead both have built-in deliverability dashboards
From Cactus: Cactus manages deliverability infrastructure for clients running high-volume cold email — we've maintained 90%+ inbox placement rates on campaigns at 1,000+ emails/day through rigorous domain hygiene and monitoring.
Cactus Marketing embeds with B2B tech startups to turn strategy into pipeline. We've worked with 60+ companies, supported 12 exits, and contributed to $7B+ in client valuations.
Book a free 30-minute call — we'll give you a concrete plan for your situation.
Book a free strategy call →How do I write a cold email that gets replies?
Write one sentence that's specifically about them, one sentence on their problem, one on your solution, and one CTA. The email should be under 80 words, reference something real about their company, and ask a yes-or-no question at the end.
How do I find leads for cold email?
Start with Apollo.io or LinkedIn Sales Navigator to build a list from your ICP criteria. Enrich it through Clay or a waterfall of data providers for verified emails. Target trigger events — recent funding, new hires, job postings — to catch companies in active buying moments.
What is a good cold email reply rate?
A good cold email reply rate is 3–8% for a broad ICP campaign and 8–15% for a highly personalized, trigger-event-based campaign. Anything above 15% with meaningful volume is excellent. Below 2% means something fundamental is wrong — ICP, targeting, or the email itself.
How many cold emails should I send per day?
New domains should warm up for 2–4 weeks before sending real campaigns, starting at 10–20 emails/day and capping out at 30–50/day per domain for sustained campaigns. With multiple warmed domains running in rotation, total volume can reach 500–2,000+/day without domain damage.