Email Marketing Deliverability
Email Deliverability Explained: How to Reach the Inbox
What happens between pressing send and landing somewhere, and the five signals mailbox providers weigh when deciding where.
On this page
The short version
- Delivered and inboxed are different things. Your platform reports acceptance by the receiving server, which counts a message filed straight into spam as delivered.
- Reputation is attached to your sending domain and your sending IP, and it is built from how recipients behave rather than from what your email says.
- The lever with the largest and fastest effect is almost always list hygiene, because it changes the engagement ratios everything else is scored on.
Between pressing send and a message appearing somewhere, a chain of decisions happens that most senders never see. Understanding the order is what stops you fixing the wrong thing — rewriting copy when the problem is a DNS record, or buying a monitoring tool when the problem is forty thousand addresses that have never opened anything.
The chain is short, and each step can only be reached if the previous one passed.
The five signals, in the order they are weighed
Mailbox providers do not publish their scoring, but the inputs are well established from their own documentation and from what senders can observe. They fall into five groups, and the order matters because a failure high in the list makes everything below it irrelevant.
What is actually being scored
| Signal | What it means | How fast you can change it |
|---|---|---|
| Authentication | SPF, DKIM and DMARC align and pass | Hours — it is a DNS change |
| Complaint rate | Recipients pressing the spam button | Days, by suppressing the wrong segments |
| Engagement | Opens, clicks, replies, moves out of spam | Weeks — it is a ratio across your list |
| List quality | Bounces, unknown users, spam traps | Days, by cleaning |
| Content and infrastructure | Links, images, sending patterns, IP history | Varies |
Authentication is first because it is binary and cheap. There is no reason to be diagnosing engagement while DKIM is unaligned.
Reputation is about behaviour, not wording
The persistent folk model of spam filtering is that certain words trip it. That was true enough two decades ago and is now a small part of a much larger picture. What providers principally measure is how recipients treat mail from you: whether they open it, whether they reply, whether they delete it unread, and whether they mark it as spam.
This is why two senders can transmit near-identical content and land in different places. The message is the same; the history attached to the sending domain is not.
It also explains why deliverability problems feel sudden. Reputation degrades gradually as engagement ratios worsen, and then crosses a threshold where filtering changes — so the visible symptom arrives long after the cause started.
Domain and IP
Reputation attaches to two things: the domain in your from address, and the IP address the mail was sent from. On shared sending infrastructure — which is most platforms at most price points — the IP is shared with other senders, so your domain reputation is doing most of the work for you.
That is generally fine and occasionally not. A shared pool containing a badly behaved sender affects everyone on it, which is one of the few genuine arguments for a dedicated IP. The counter-argument is stronger for most senders: a dedicated IP needs enough consistent volume to maintain a reputation of its own, and below that threshold you are worse off alone than in a well-managed pool.
Where to start when placement drops
In order, and stopping as soon as you find something: check that authentication still passes and still aligns, because DNS changes made for other reasons break it regularly. Check complaint rate per segment rather than overall — one bad segment can carry the whole account. Check what changed in the last fortnight, including things that were not email changes.
Only after those three is it worth looking at content, and by then you will usually have found the cause.
The baseline every sender should hold
- SPF, DKIM and DMARC published, passing and aligned to your domain
- Bounce suppression automatic and verified working
- Complaint rate under 0.1%, checked per segment
- A defined inactivity threshold, and something that acts on it
- One-click unsubscribe honoured immediately
- The list-unsubscribe header set on every commercial send
Frequently asked questions
How do I find out where my mail is actually landing?
Seed testing gives you placement across a panel of accounts, and Google Postmaster Tools gives you reputation and complaint data for your own domain at Gmail. Neither shows placement for your real list, which nobody can see — treat both as trend indicators.
Does buying a dedicated IP fix deliverability?
Only if the problem was the shared pool, which it usually is not. A dedicated IP needs steady volume to hold a reputation; below that it makes you more fragile rather than less.
How long does recovery take?
Weeks rather than days, because the signals are ratios accumulated over time. The fastest lever is removing unengaged addresses, since it changes the denominator on every engagement ratio immediately.