How to diagnose placement issues by ISP?

Your email's overall metrics look fine, but you notice Gmail recipients barely open while Hotmail crushes it. That tells you the problem isn't your list or your content. It's ISP-specific. Here's how to dig in.

Step 1: Segment your metrics by ISP. Break down your opens, clicks, and bounces by mailbox provider (Gmail, Outlook, Yahoo, iCloud, etc.). Most ESPs let you pivot your analytics this way. You'll quickly spot which ISP is the underperformer. If Gmail opens are 15 percentage points lower than your average, that's your starting point.

Step 2: Check the provider's tools. Gmail offers Postmaster Tools (free, requires domain verification). Microsoft has SNDS for IP reputation data. These show you authentication issues, spam rate, and complaint rate specific to that provider. If authentication's failing there, you've found your problem. Fix it and move on.

Step 3: Run a targeted seed test. Use a tool like GlockApps and focus your test on the problem provider. Send the same email to Gmail seeds and Hotmail seeds. See where it lands (inbox vs. spam). If Gmail goes to spam but Hotmail goes to inbox, it's a filtering issue specific to Gmail's algorithm, not your list. If both go to spam, it's reputation. That distinction matters because the fix is different.

Step 4: Investigate what changed. Did you add a new IP? Change your authentication? Send to a larger segment? Pull up your email history. Most ISP-specific problems correlate with something recent. Was Gmail fine last month? What's different about this campaign or sending pattern?

And once you know it's a Gmail issue (not all ISPs), your fix options become clearer. It might be SPF alignment, or it might be your sender reputation with that specific provider, or it might be content that Gmail's filter scrutinizes differently.

Next step: Log into your ESP's analytics right now and pull your open rates by ISP. You'll spot the problem provider immediately. Then run a seed test on that one ISP using GlockApps and you'll know whether it's inbox placement or something else.