Deliverability problems get diagnosed backwards. Opens drop, someone panics about subject lines, and three weeks later the real issue, a reputation problem that started when the list got blasted in November, is still there doing damage.
Deliverability is not one setting. It is a stack of things that have to be right at the same time, and when inbox placement slips, the job is to find which layer broke, not to change everything at once and hope. This is the diagnostic we run, in order, from the cheapest checks to the deepest. Work it top to bottom. Most problems are found and fixed in the first three sections.
A note on why this matters more at scale. At £1M+ you are sending enough volume that mailbox providers have a detailed opinion of you. That opinion compounds. A small reputation problem left alone becomes a large one, and by the time it shows up in revenue it has usually been building for weeks.
First, confirm you actually have a problem
Before diagnosing anything, rule out the false alarm. Half the deliverability panics we see are misreads.
Open rate is not a reliable signal anymore. Since Apple’s Mail Privacy Protection started auto-opening mail, your open rate is inflated by bot opens and cannot be read as a clean measure of inbox placement. A “dropping” open rate might be Apple changing how it prefetches, not your mail hitting spam. Do not start surgery based on opens alone.
What to trust instead: click rate, click-to-open patterns over time, and above all any change in revenue per recipient. If clicks and revenue are steady and only opens moved, you probably do not have a deliverability problem. If clicks and revenue are sliding together, you do.
Confirm placement directly. Use seed testing or a placement tool to see where your mail actually lands across Gmail, Outlook and Apple, and check your engagement broken down by provider inside Klaviyo. If Gmail is fine and Outlook is in the bin, that narrows the diagnosis immediately. Provider-specific problems have provider-specific causes.
Only once you have confirmed a real placement or engagement decline do you work the rest of this list.
Layer 1: Authentication and infrastructure
The foundation. If this is wrong nothing above it works, and misconfiguration here sends you straight to spam regardless of how good your content is.
Check your authentication is complete and passing. SPF, DKIM and DMARC all need to be set and aligned. Since Google and Yahoo tightened bulk sender rules in 2024, DMARC is effectively mandatory for anyone sending real volume. Missing or failing authentication is the single fastest route to the spam folder, and it is invisible unless you look.
Check you are on a dedicated sending domain, not sending from your root domain. Your sending subdomain isolates your email reputation from your main domain, and gives you a clean surface to build and protect sender reputation. Verify it is set up and that the DNS records are actually live.
Check your dedicated vs shared IP situation. At £1M+ volume you may be on a dedicated IP, which means the reputation is entirely yours to build and yours to damage. If you are on shared infrastructure, other senders affect you. Know which you are on before you interpret any reputation signal.
If authentication is failing, stop here and fix it. Everything below assumes the foundation is sound.
Layer 2: List quality and hygiene
The most common real cause of deliverability decay at scale, and the one brands resist fixing because it means sending to fewer people.
Check what you are actually sending to. Mailbox providers judge you on how the recipients behave. Sending to people who never open or click drags every signal down, and past a certain point of accumulated dead weight, providers start routing even your good mail to spam. The size of your list is not the asset. The engagement of your list is.
Check for a sunset policy, and check it is actually running. You need automated suppression of chronically unengaged profiles, and most brands either never built it or built it and quietly stopped enforcing it. A profile that has not opened or clicked in six months and ignored a winback is hurting you every time you send to it.
Check how you are acquiring addresses. Aggressive popups, purchased lists, and any acquisition that pulls in low-intent or fake addresses feed spam traps and complaints directly into your programme. A spike in list growth that preceded a deliverability drop is a prime suspect. Trace it.
Check your list against your complaint and bounce rates. Rising hard bounces or spam complaints after list growth almost always means the new addresses are the problem.
Layer 3: Sending behaviour and reputation
How you send, not just who you send to. This is where good lists still get into trouble.
Check your complaint rate against the line. Google’s own threshold is a spam complaint rate below 0.1%, and drifting above it degrades placement for your whole programme. If you are near or over it, that is a five-alarm finding and it points back to list quality and frequency.
Check your sending consistency. Providers trust steady, predictable senders. A brand that goes quiet for three weeks and then fires a Black Friday blast at the full list looks, to a mailbox provider, like a compromised account. Volume spikes are a reputation risk in themselves. If your sending is erratic, that alone can explain placement problems.
Check whether you warmed up recent volume increases. If you ramped sending fast, onto a new IP or domain or just a much bigger list, without a gradual warm-up, you may have burned reputation on the way up. Reputation is built slowly and lost quickly.
Check your frequency against your engagement tiers. Over-mailing the unengaged tail is one of the most common ways to manufacture a complaint problem. If you are not segmenting frequency by engagement, that is both a revenue and a deliverability issue.
Layer 4: Content and placement signals
Last, because it is the layer people reach for first and it is rarely the root cause on its own.
Check your Gmail tab placement separately from spam placement. Landing in Promotions is not the same as landing in spam. Promotions is a normal category and often fine. If your problem is Promotions placement specifically, that is a narrower issue with its own fixes, and we cover it in depth in our Promotions tab guide rather than here.
Check your content signal load on the emails that matter. Heavy image-only emails, broken links, huge file sizes and spammy construction can tip borderline mail. This matters most for the emails you need in the primary inbox, your lifecycle and relationship sends. Reserve plain, light, personal formatting for those.
Check for broken or suspicious links. Link shorteners, links to flagged domains, or a tracking domain that is not properly configured can all trip filters. Verify your click-tracking domain is set up and clean.
Priority order
Work the diagnosis in this sequence. The order is deliberate, cheapest and most common causes first.
- Confirm the problem is real. Check clicks and revenue, not just opens. Confirm placement by provider.
- Verify authentication and infrastructure. SPF, DKIM, DMARC, dedicated sending domain. Foundation before anything else.
- Audit list quality. Engagement over size, sunset policy running, acquisition sources clean.
- Examine sending behaviour. Complaint rate under 0.1%, consistent volume, frequency segmented by engagement.
- Review content and placement signals last. Separate Promotions from spam, lighten the emails that need the primary inbox.
The pattern holds across every deliverability problem we audit: the cause is almost always lower in the stack than where people start looking. Subject lines and templates get blamed for what is really an authentication gap or a list-hygiene problem. Diagnose from the foundation up and you find it faster.
Advanced monitoring sits on top of this once the basics are clean: Google Postmaster Tools for ongoing reputation and complaint visibility, BIMI once DMARC is enforced, and per-provider engagement dashboards so a decline shows up in days rather than in the revenue report. Those are worth building once the four layers above are solid. If your opens or revenue have moved and you cannot tell which layer is the cause, that is exactly the diagnosis worth running properly before you change anything.
Free deliverability audit
If you want a second pair of eyes on your inbox placement, or your opens and revenue have moved and you cannot pin down why, book a free 30-minute audit with us. We will work the stack from authentication up and show you which layer is actually costing you placement.
Or if you want to talk through strategy first: Book a strategy call
Jamie Watkins, Director at UndergroundEcom
[LinkedIn] | [UndergroundEcom.com]
