Testing email filtering behaviors across platforms

A common mistake I see brands make is relying on their own inboxes to test email campaigns. But just because it looks great on your device doesn’t mean it will for your customers. What's often not taken into consideration is how your campaigns render across the 60+ platforms and devices your customers might be viewing your campaigns on. This means that while you and even your team might see a beautifully designed, well-put-together campaign, your customers might be seeing a completely skewed design. Not quite the outcome you'd like... And without proper testing, that beautifully designed campaign could appear distorted, unreadable, or even completely broken for some recipients. Dark mode is a perfect example. It's estimated that around 40% of users have dark mode enabled on their devices, yet most brands don’t test how their emails render in dark mode. The result? Logos that disappear, unreadable text, and broken design elements that ruin the user experience. Internally, we use Litmus to check formatting, links, and deliverability before sending and while this is our go-to, Sinch Email on Acid also does the trick and is much more cost-effective for brands. To give you an idea, here's what you can do using a third-party tool like Litmus or Emails on Acid: ✔️ Ensure emails display correctly, including in dark mode ✔️ Make sure all links work ✔️ Confirm compatibility across 60+ devices ✔️ Prevent email clipping, especially in Gmail (102KB limit) ✔️ Minimise human error by testing beyond just your inbox ✔️ Validate mobile responsiveness ✔️ Provide proper authentication to avoid being flagged as spam ✔️ Monitor for blocklists and spam placements ✔️ Check email load times to avoid slow rendering ✔️ Review accessibility compliance (contrast, font size, readability) I’m still waiting for an ESP to integrate this functionality directly - it would be a game changer. Until then, proper testing is non-negotiable.

Most people blame deliverability issues on the basics: • DNS • warmup • spam words Those matter but they’re not why many campaigns quietly degrade. Modern spam filtering is behavioral. And these are the signals most senders miss. 1. Reusing the Same Copy Across Too Many Inboxes Even if the copy is “good,” repeating it across dozens of inboxes creates a recognizable pattern. Same layout. Same spacing. Same CTA every time. Inbox providers don’t just read content. They analyze similarity at scale. Small variations matter more than most people think. 2. Hitting Accept-All Domains Too Fast Sending too much volume to accept-all domains early hurts reputation fast because engagement is unpredictable and bounces surface late. - Limit accept-all volume - Spread it across more inboxes - Ramp it slower than verified emails 3. Old, Inactive Inboxes on the Same Domain This one’s easy to miss. Even inboxes that aren’t sending can hurt you if they’re inactive on the same domain: • no logins • no replies • no human behavior Providers look at domain-level activity, not just the sending inbox. 4. Sending links in cold email I don’t recommend any links in cold emails. Zero. They hurt deliverability and add friction. If needed, add links only after a reply or in follow-ups. Plain text + a simple question works best early on. 5. Sending at the Exact Same Time Every Day Perfect consistency looks… unnatural. Human senders don’t operate like cron jobs. Tiny time variations help: • hour ranges • day-to-day shifts • randomized scheduling It’s a small change with an outsized impact. Deliverability today isn’t about avoiding words. It’s about avoiding patterns. If your setup looks automated at scale, filters will treat it that way. Fixing these five things won’t show up on a dashboard but your inbox placement will tell the story.

Most people pick tools based on price or features. Very few test whether their infrastructure delivers. Here's a framework for evaluating your sending stack: 1️⃣ Multi-SMTP relay test - Add multiple SMTP relays to your email platform (SendGrid, Sinch Mailgun, Postmark, SMTP, custom SMTP) - Send the same campaign through all of them - Each email rotates across different IPs - Compare inbox rates per relay ↳ You'll quickly see which infrastructure actually performs 2️⃣ Weekly inbox placement monitoring - Don't just test once and assume you're fine - Shared IPs change every week - ISP algorithms update constantly - Set it up on automatic weekly cadence 3️⃣ Blacklist scanning on all layers - Domain IPs - SMTP relay IPs - Links and Images IPs - Secondary domain IPs - If you're not scanning all of them, you have blind spots 4️⃣ Provider-level performance tracking - Break down your metrics by Gmail vs. Outlook vs. Yahoo - Each ISP has different filtering behaviors - You might inbox on Gmail and spam on Outlook from the same IP - Optimize per provider, not in aggregate A tool that performs today might not perform next week. Shared IPs, policy changes, provider updates - everything shifts. Your infrastructure is only as good as your last test. How often are you testing yours?

My deliverability dashboard said 100%. But it was a lie. Green check marks everywhere. Then a prospect replied: "Found your email in my spam folder." Damn. So I started digging.  Turns out it wasn't just one email landing in spam.  My tools never flagged it. Most teams rely on open rates as their deliverability metric. But: → Spam filters hate the tracking pixel → Some providers now blocks it entirely → It can't tell you if you landed in inbox or elsewhere So you check your outbound platform's deliverability score instead. Green lights, high scores, you move on. But those scores come from a closed warmup pool. Emails bouncing between mailboxes inside the same system. It's not the real world. The fix is called inbox placement testing. You send to a group of real monitored mailboxes across multiple providers outside the warmup pool. Each one reports back exactly where your email landed. No guessing. I moved to Mailpool for this.  They have ~5,000 monitored test mailboxes rotated monthly. You pick which providers to test against, send your email, and get a dashboard showing exactly where you landed per provider. My results were ugly. But at least they were real. Now I have a better understanding and control. If cold email is one of your acquisition channels and you don't have inbox placement testing running, you're in troubles. Open rates won't save you.  Warmup scores won't save you. Real emails to real mailboxes in real conditions. That's the only metric that counts. Give Mailpool a try, you might be surprised with what you find! What's your current setup for monitoring deliverability beyond open rates?

Your email platform says 98% delivered. Your open rate says 4%. Those 96% didn't ignore you. They never saw you. 𝗧𝗵𝗲 𝗽𝗿𝗼𝗯𝗹𝗲𝗺: "Delivered" means the server accepted it. It doesn't mean: → It reached the inbox → It avoided the spam folder → It wasn't silently discarded → A human ever saw it Your ESP can't tell the difference. But your pipeline can. 𝗧𝗵𝗲 𝗻𝘂𝗺𝗯𝗲𝗿𝘀: We tracked inbox placement across 4.2 million B2B emails. → ESP reported delivery rate: 97.3% → Actual inbox placement: 61.2% → Spam folder: 24.8% → Silently discarded: 11.3% 36% of your "delivered" emails are invisible. 𝗪𝗵𝘆 𝘁𝗵𝗶𝘀 𝗵𝗮𝗽𝗽𝗲𝗻𝘀: → Gmail uses engagement history to filter new senders → Microsoft 365 scores sender reputation per recipient → Corporate filters learn from employee behavior → First email performance affects all future emails → Low engagement trains filters to deprioritize you 𝗨𝗻𝗰𝗼𝗺𝗺𝗼𝗻 𝗳𝗮𝗰𝘁𝘀: → Gmail's spam filter blocks 10 million emails per minute → Microsoft flags 49% of first-time cold senders → Corporate IT can see you're in spam. They won't tell you. → Engagement-based filtering means one bad campaign poisons the next → Warm domains with no engagement still get filtered 𝗧𝗵𝗲 𝗰𝗼𝘀𝘁: A 50,000-email campaign with 36% invisible delivery: → 18,000 emails no one ever saw → $270,000 in wasted pipeline (at $15/contact) → Damaged sender reputation → Lower placement on next campaign 𝗧𝗵𝗲 𝗳𝗶𝘅: → Use seed testing to measure actual inbox placement → Segment by email provider (Gmail vs Microsoft vs corporate) → Warm new domains with engaged recipients first → Monitor engagement by domain, not aggregate → Clean disengaged contacts before they hurt your reputation "Delivered" is a technical status. "Inbox" is what matters. We verify inbox placement. Not just server acceptance. 😁

Smartlead and Instantly.ai integrating inbox spam tests has changed how we are monitoring email deliverability at Growth Engine X. Here’s an overview of what we are doing. First, for those that don’t know, an inbox placement test is when you send a test email to a group of emails that will report back to you where the email landing. Primary, Promotions, or Spam? Now it’s not perfect because your spam filter learns from what emails you mark as spam and obviously these inboxes have never marked anything as spam. So you should know that but I still find these tests useful enough to run now that they are integrated with the platforms. We are now running a test daily at 11 pm EST on all active campaigns and getting the inbox placements. Then, every Tuesday and Friday we will use an internal API to call to list out all inboxes that landed in spam, remove them from campaigns, and tag them so we don’t use them again. We always keep extra inboxes warming for our customers so we will push in the extra fresh inboxes as we remove the ones landing in spam. Everything can be done automatically except selecting the inboxes we will use to replace the ones in spam which I’m not sure is even worth automating. Hopefully, this gives something to think about for those that also don’t use open tracking and need a way to track email deliverability in their cold email campaigns!

Your spam placement test is lying to you. Because not all placement tests are created equal: I see people running one GlockApps test and assuming their deliverability is solid. Then they send a campaign and get 0 replies. Here’s why: 1️⃣ GlockApps ≠ Real Cold Email Traffic 🔹 GlockApps and MailReach log into a mailbox and send test emails to seed addresses. 🔹 But they’re internal tests, not real outbound emails. Outlook and Google treat these differently than emails sent to prospects. 2️⃣ Smartlead Smart Delivery & Instantly.ai ≠ Traditional Testing 🔹 These tools test placement directly from their infrastructure—so they mimic real-world sending conditions better (if you will send your outreach from the same tools) 🔹 If your email lands in spam in these tests, it’s likely to land in spam when sending to prospects. 3️⃣ Your "Inbox Rate" is Misleading 🔹 Getting 80% inbox placement on GlockApps? That doesn’t mean 80% of your cold emails are hitting primary inboxes. Because Glock’s seed emails aren’t your actual prospects—they don’t behave the same way. 🔹 Your real inbox rate depends on engagement, past interactions, and sender history. How to Actually Test Deliverability? ✅ Run tests across multiple platforms (GlockApps, MailReach, Instantly.ai, Smartlead). ✅ Compare results: If Smartlead shows spam but Glock shows inbox, trust Smartlead’s result since they’re the ones with the sending infra itself. Same applies for any other sequencer with placement testing. Also keep in mind the tenants, usually platforms will have all their domains in single msft tenants or google workspace admin — meaning that results are extremely likely to duplicate across the rest of the domains. Most senders don’t know their true deliverability because they only trust one test. Stop assuming. Start testing smarter. 💬 How are you testing your inbox placement right now? Drop a comment—I’ll break it down.

If your cold emails are getting 0 replies high chances AI is deleting your emails. Gmail’s AI now looks at how you send not just what you send. We learned this the hard way. Here are 11 deliverability signals you need to watch for: 1. Dedicated SMTP on VPS We ditched shared sending tools. Set up Postfix on our own VPS with a dedicated IP. → Gmail saw a clean sender identity. → No third-party footprints. → Inbox rate jumped. 2. Full DNS config (beyond the basics) Everyone sets up SPF, DKIM, DMARC. But that’s not enough. We added: - PTR records (reverse DNS) - FCrDNS (forward-confirmed reverse DNS) These verify that your IP truly belongs to your domain. Gmail checks this. 3. Manual warm-up, slow scaling We started by sending just 5–10 plain-text emails/day. No links. No HTML. No tools. Just… real emails. Once reply rates held steady → we scaled gradually over 45 days. 4. Rate-limiting + greylisting config We made our infrastructure mimic human behavior: - Slow response to bursts - Accept retries from real mail servers → Gmail uses this to separate humans from bots. 5. Live monitoring dashboard We now track: - Spam complaint loops (via Spamcop, AOL, etc.) - IP and domain reputation - Open / reply/bounce patterns - Domain “temperature” by ESP ⚠ One red flag → system pauses, reroutes, resets warm-up. 6. IP rotation logic When one IP gets hot or flagged → we rotate. Reputation protected. Deliverability stable. 7. Consistent send windows No 2 AM email blasts. We trained Gmail to expect behavior that looks… human. 8. Inbox placement testing We test inbox placement daily across Gmail, Outlook, Zoho, Proton, and more. It’s the only way to catch silent spam filtering. 9. Clean, human copy No AI polish. No “quick question” openers. We used lowercase subjects, occasional typos, and a human-like tone. Gmail flags anything that looks like a mass template. 10. Avoided known spam triggers We stripped: - Link shorteners - Scheduling tools - Spammy CTAs Gmail flags predictable phrases fast. 11. Used Warmy.io - Email channel. Reliable. to keep everything stable We now run Warmy alongside our infrastructure. It detects deliverability risks → AI crafts optimized warmup sequences - Auto-warmup across 150K emails/month in 30+ languages - Human takes over once inbox placement stabilizes It’s the quiet system in the background constantly making sure we stay clean, warm, and consistent. Google doesn’t care how clever your hook is. It cares if your server looks is clean. ⸻ Send this to your SDR before they write the next campaign.