How do you build a deliverability diagnostic checklist?

Something's off. Open rates dipped, a client mentioned their emails aren't arriving, or your ESP dashboard is showing an uptick in bounces. You know something broke, but you're not sure where to start looking. That's exactly when a deliverability diagnostic checklist earns its keep.

A good checklist doesn't just list what to check. It tells you what order to check things, what each result means, and what to do next. Here's how to build one that actually works.

Step 1: Start with the technical layer

Before anything else, confirm your authentication is passing. A broken SPF record, a misaligned DKIM signature, or a DMARC policy that suddenly shifted to p=reject can tank delivery overnight without any reputation change at all. These are the easiest problems to confirm and the fastest to fix.

• SPF, DKIM, and DMARC all passing and aligned?
• DNS records intact and propagated correctly?
• Sending infrastructure online and healthy?

You can run these checks in minutes with RME's free SPF checker and DKIM checker. If anything's broken here, fix it before moving on. There's no point diagnosing reputation if your authentication isn't solid.

Step 2: Check your reputation signals

But once you've confirmed the technical layer is clean, look at how mailbox providers see your sending identity. This is where the real story usually lives.

• Is your domain or IP on any major blocklists? (Check via our free blocklist checker)
• What does Gmail Postmaster Tools show for your domain reputation and spam rate?
• What does Microsoft 365 SNDS report for your sending IP?
• Are feedback loop complaints spiking at any provider?

Low reputation at Gmail and a blocklist hit are very different problems with very different fixes. Knowing which one you're dealing with shapes your entire recovery approach.

Step 3: Look at your sending behavior

Technical checks and reputation checks tell you the current state. Behavioral checks tell you why things changed. Go back through your recent send history and look for anything that shifted.

• Did you send a significantly higher volume than usual?
• Did your list composition change? (A new import, a reactivation campaign, a purchased segment?)
• Did engagement patterns shift before the problem appeared?
• Did you change content, subject line style, or sending frequency?

Deliverability problems rarely appear out of nowhere. There's almost always a trigger event a few days earlier. Building a timeline of what changed and when is one of the most useful things you can do during a diagnosis. It's what separates correlation from actual causation.

Step 4: Analyse your bounce data

Bounce codes are blunt instruments but they tell you a lot. Soft bounces mean temporary rejection. Hard bounces mean permanent failure. A sudden spike in either deserves its own section of your checklist.

• What are the specific bounce codes and error messages coming back?
• Are bounces concentrated at one provider (Gmail vs. Outlook vs. everyone)?
• Are they hitting a specific segment, domain, or list source?
• Has the bounce pattern changed over the last 7 or 30 days?

Provider-specific patterns are especially useful. If you're getting deferrals only at Gmail but sailing through at Outlook, that narrows your diagnosis considerably. If you're failing everywhere, the issue is probably technical or your domain reputation is widely affected.

Building it into a living document

Still a checklist you run once and never look at again isn't much of a diagnostic tool. The goal is to build something your whole team can run consistently, so that when something breaks, anyone can start in the right place without recreating the process from memory.

Each section of your checklist should have three things: what to check, what tool or source to use, and what a bad result means. "SPF passing" is not enough. "SPF passing via RME SPF Checker or email header review. If failing, look for recent DNS changes or added sending services." That's a checklist entry worth running.

If you're building this from scratch and want to make sure nothing's missing, our SOS hotline is free and we're happy to walk through your specific setup with you. No pitch, just help.