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.

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.