Email Deliverability: What It Is, What Breaks It, and How to Fix It

Most people discover email deliverability the same way: a campaign that used to work stops working, nothing obvious changed, and the reporting says everything was delivered. This guide covers what's actually happening in that gap, and what you can do about it.

What is email deliverability?

Email deliverability is the rate at which your messages reach recipients' inboxes rather than being blocked, bounced, or filtered into spam. It is distinct from delivery — a message can be successfully delivered to the receiving server and still never reach the inbox.

That distinction is the entire subject. Your sending platform reports on delivery, because delivery is the last thing it can observe. Deliverability happens after the handoff, inside infrastructure you have no visibility into, and it's where the outcome is actually decided.

Delivered vs. inboxed: why your dashboard lies

When your email tool reports a 99% delivery rate, it means 99% of messages were accepted by the receiving mail server without a bounce. It says nothing about where those messages went afterwards.

There are roughly four destinations:

| Outcome | What your dashboard says | What actually happened | |---|---|---| | Inbox | Delivered | The message reached the primary inbox | | Spam folder | Delivered | Accepted, then filtered. Almost never read | | Promotions/Updates tab | Delivered | Accepted and inboxed, but in a tab most people don't check | | Silently dropped | Delivered | Accepted and then discarded without a bounce |

That last row surprises people. Some providers accept a message to avoid confirming to a potential spammer whether an address exists, then discard it. The sender sees a clean delivery confirmation for a message that no longer exists anywhere.

The practical consequence: you cannot measure deliverability from your sending tool's reporting. You have to measure it from the receiving side, which we'll come to.

What determines whether you reach the inbox

Filtering decisions come down to five categories, roughly in the order providers weight them.

1. Authentication

Three DNS records tell receiving servers that mail claiming to be from your domain actually is:

SPF lists which servers are allowed to send as your domain
DKIM cryptographically signs your messages so tampering is detectable
DMARC tells receivers what to do when SPF or DKIM fails, and where to send reports

Authentication is a gate, not a ranking factor. Passing it doesn't earn you inbox placement; failing it costs you almost everything else. Our deliverability checklist walks through each record and how to verify yours.

2. Sender reputation

Providers maintain a running assessment of your domain, your sending IP, and in some cases the individual mailbox. It's built from complaint rates, engagement, bounce rates, spam-trap hits, and consistency of sending volume. Reputation is the single largest input, and it's cumulative — which is why recovery is slower than damage.

3. Recipient engagement

Opens, replies, forwards, and messages moved out of spam are positive. Deletions without opening, and especially spam complaints, are negative. For cold outreach this is the structurally hard part: cold email generates weaker engagement than any other category of legitimate mail, which is why senders need something else diluting those signals.

4. Content and formatting

Less decisive than folklore suggests, but not irrelevant:

Tracking pixels and link-tracking redirect domains are both recognisable filter signals
Heavy HTML, image-only messages, and link-dense bodies score worse than plain text
Shortened URLs and freshly registered link domains are treated with suspicion
Identical message bodies sent at volume are detectable regardless of merge fields

5. Infrastructure and history

Domain age, whether your domain or IP appears on a blocklist, whether the domain has prior abuse history, and — for new mailboxes — whether there's any sending history at all. A domain registered last week has no reputation to draw on and gets the default treatment for unknown senders, which is suspicion.

The Google and Yahoo sender requirements

In February 2024, Google and Yahoo introduced enforced requirements for bulk senders. If you send to Gmail or Yahoo addresses at any meaningful volume, these are no longer best practices — they're conditions of delivery.

The requirements for bulk senders are:

SPF, DKIM, and DMARC all configured, with DMARC alignment. A p=none policy satisfies the requirement, but it provides no enforcement — it's the floor, not the goal.
One-click unsubscribe in marketing messages, honoured within two days.
Spam complaint rate below 0.3%, measured in Google Postmaster Tools. Above that threshold, filtering begins regardless of everything else.

The 0.3% number deserves attention because it is much stricter than it sounds. Three complaints per thousand messages is a threshold you can cross with a single badly targeted campaign. And complaint rate is trailing — you find out you crossed it after the damage is done.

If you send cold outreach, you're probably below the formal bulk-sender volume threshold. Treat the requirements as applying to you anyway. The thresholds define what "normal" looks like to the filter, and being outside normal is the thing you're trying to avoid.

How to actually measure deliverability

Since your sending dashboard can't tell you, there are three approaches, in increasing order of usefulness.

Open rates as a proxy. Weak, and getting weaker. Apple Mail Privacy Protection pre-loads images, which inflates opens for a substantial share of recipients. A falling open rate tells you something is wrong; a healthy one doesn't tell you things are right.

Google Postmaster Tools. Free, and the only direct window into how Gmail specifically sees your domain — domain reputation, spam rate, and authentication pass rates. It requires DNS verification and only produces data above a volume threshold, so small senders often see empty charts. Set it up anyway; you want the history when you need it.

Seed list / inbox placement testing. You send a message to a set of test mailboxes across different providers and observe where each one lands. This is the only method that directly answers "inbox, spam, or tab, per provider". It's a point-in-time sample rather than continuous monitoring, but it's the closest thing to ground truth available.

A reasonable cadence: Postmaster Tools reviewed weekly, a placement test before any significant campaign and after any change to your sending setup.

The most common causes, in the order you should check them

When deliverability drops, resist the urge to rewrite your copy first. Copy is rarely the cause and it's the slowest thing to test. Work through this order instead.

1. Check authentication is still passing. DNS changes, domain transfers, and adding a new sending tool all break SPF and DKIM regularly. This takes two minutes to rule out.

2. Check whether you're on a blocklist. If your domain or sending IP is listed, nothing else matters until you're delisted — and continuing to send while listed makes it worse. Most blocklists publish a delisting process.

3. Check for a volume change. Did you add mailboxes, increase daily sends, or resume after a pause? Volume spikes are the most common self-inflicted cause, and pauses are the sneakiest version: stopping for three weeks and resuming at your previous rate reads as a spike, no matter how established you were before.

4. Check your bounce rate. A rising hard-bounce rate means list quality has degraded, and providers read repeated sends to nonexistent addresses as evidence you acquired the list rather than earned it.

5. Check complaint rate in Postmaster Tools. If you've crossed 0.3%, targeting is the problem, not deliverability tactics.

6. Then look at content. Tracking pixels, link-tracking domains, image-heavy layouts, and near-identical bodies at volume.

Cold email deliverability is a different problem

Everything above applies to all email. Cold outreach adds a structural difficulty worth naming: the engagement signals that protect your reputation are exactly the signals cold email doesn't produce.

A newsletter to an opted-in list generates opens and clicks from people who asked for it. A cold email to a stranger generates, at best, indifference. The filter sees the same absence of engagement in both cases and draws the obvious conclusion.

The senders who make cold outreach work sustainably deal with this in four ways:

Separate domains. Cold outreach runs on a domain distinct from the one used for real business correspondence, so a deliverability problem can't damage the address customers reply to. Usually a lookalike domain of the main one.

Multiple mailboxes at low volume each. Rather than one mailbox sending 200 a day, several mailboxes sending 30–40 each. Per-mailbox volume stays inside normal-human range.

Warmup running continuously. A stream of ordinary, engaged, two-directional messages that dilutes the poor engagement profile of the campaign traffic. Not a one-time setup before launch — running the whole time. See our guide to SMTP warmup for the mechanics and a realistic ramp schedule.

Tighter targeting than feels necessary. The 0.3% complaint threshold is unforgiving, and the cheapest way to stay under it is to send to fewer, better-matched people.

Fixes that actually move the needle

In rough order of impact per unit of effort:

Fix authentication properly — SPF, DKIM, and DMARC with alignment, not just present
Get delisted if you're on any blocklist, before doing anything else
Reduce volume per mailbox rather than adding more sends to a struggling one
Run warmup continuously, including during campaigns
Remove link tracking and tracking pixels from cold outreach specifically
Send plain text for cold email; save HTML for people who opted in
Clean your list — verify addresses before sending, remove hard bounces immediately
Age your domains — buy them weeks before you need them, not days
Tighten targeting to reduce complaints at the source

Note what isn't on the list: rewriting subject lines to avoid "spam words". Modern filters are not keyword matchers, and there is no list of forbidden words that meaningfully drives placement. Time spent there is time not spent on the nine items above.

Frequently asked questions

What is a good email deliverability rate? For opted-in mail, 95%+ inbox placement is achievable and anything below 90% warrants investigation. For cold outreach, sustained inbox placement above 80% is a reasonable target and above 90% is very good. Rates quoted without specifying inbox placement rather than delivery are usually measuring the wrong thing.

Does email deliverability differ by provider? Substantially. The same message can inbox at Gmail and land in spam at Outlook. Microsoft weights domain-level reputation more heavily and is less forgiving once a sender is filtered. If your recipients are predominantly on one provider, test against that provider specifically.

How long does it take to recover from bad deliverability? Weeks, not days, and longer than it took to damage. Reputation is calculated over a trailing window, so recovery requires accumulating enough good history to outweigh the bad. Reduce volume, fix the underlying cause, keep warmup running, and expect three to six weeks before placement meaningfully improves.

Can I improve deliverability without reducing volume? Sometimes, if the cause is authentication or a blocklist. If the cause is reputation or complaints, no — volume reduction is the mechanism, not a side effect. Sending more from a struggling mailbox is the single most common way people turn a recoverable problem into a permanent one.

Do I need a dedicated IP? Usually not, and often it's actively worse. A dedicated IP has no reputation until you build one, and building it requires sustained volume most cold outreach senders don't have. Shared IPs at a reputable provider come with established reputation. Dedicated IPs make sense above roughly 100,000 messages per month with consistent volume.

Is email deliverability the same as email delivery? No, and conflating them is the most common mistake in this area. Delivery means the receiving server accepted the message. Deliverability means it reached the inbox. A 99% delivery rate is compatible with a 20% inbox rate.

If cold outreach is where your deliverability problems live, WarmRamp handles the warmup side — continuous ramping, genuine replies, and rescuing messages from spam — for Google Workspace and Microsoft 365 mailboxes.