My deliverability dropped suddenly, how do I find the cause?

Something changed. Your open rates are down, your inbox placement looks off, and you're pretty sure it wasn't like this last week. The hard part isn't knowing something is wrong. It's figuring out where to look first.

Here's how to work through it in order, because the sequence matters.

Start with the timeline. What happened in the 48 to 72 hours before the drop? Think through every change: new list segment, new DNS record, updated email content, an IP switch, a sudden volume spike, or a new campaign type. Write it down. You'll refer back to this as you investigate.

Then check your authentication. A broken SPF or DKIM record can tank inbox placement fast, and it often happens after someone updates DNS without realizing the knock-on effects. Check that SPF, DKIM, and DMARC are all passing cleanly. Our free SPF checker takes 30 seconds and is a good first step.

Next, check the blocklists. Head to Spamhaus and run both your sending IP and your domain. A listing there can explain a sudden, sharp drop across multiple providers at once. If you want to check across many lists at once, our free blocklist checker does that too.

Pull up your postmaster tools. Gmail's Postmaster Tools will show you domain reputation and spam rate trends over time. If your domain reputation moved from "High" to "Medium" or below, that's a meaningful signal. Microsoft's SNDS does something similar for Outlook traffic. Both are free, and both will tell you things your ESP dashboard won't.

Now look at your bounces and complaints. Are you seeing new error codes you haven't seen before? A spike in 5xx rejections from a specific provider points to a reputation issue at that domain. A rise in feedback loop complaints suggests your content or audience targeting shifted in a way subscribers didn't like. Check your ESP's reporting dashboard for both, and look for patterns by mailbox provider, not just totals.

Scope the problem. Is the drop happening across all mailbox providers, or just one (say, Gmail but not Outlook)? Is it all campaigns or just a specific type? All segments or a particular audience? Narrowing the scope tells you a lot. A problem isolated to Gmail usually points to domain reputation or spam rate. A problem across all providers is more likely an authentication failure or a blocklisting.

Once you've worked through those five areas, you'll usually have a pretty clear picture. The answer is almost always one of three things: something technical broke, something changed about your sending behavior, or something changed about your audience. The fix follows once you know which one you're dealing with.

So if you're still stuck after running the checks, that's what our SOS hotline is for. It's free, and we won't pitch you anything.