Email deliverability: why good mail still lands in spam
Deliverability is the difference between a message being accepted and a message being read. Those are not the same thing, and almost every sender measures the first one.
Email deliverability is whether your message reaches the inbox rather than the spam folder, the promotions tab, or nowhere at all. It is not the same thing as delivery, and conflating the two is the most common mistake in email.
Delivery is a transport fact. The receiving server accepted the message and returned 250 OK. Your logs record a success and your dashboard turns green. Deliverability is a placement fact, and it is decided after that accept, by a filter you cannot see, using signals you mostly do not control directly.
That gap is where the whole discipline lives. A campaign can be 99.8% delivered and 40% inboxed, and nothing in a standard sending log will tell you.
Send request
Agent, campaign send
POST /v1/my/messages
reputation gate
0.711.0 = healthy domain
Scored per sending domain, every send.
spam rate vs 0.30% cliff0.28%
auth alignmentspf + dkim
bounce trend, 7dflat
complaint trend, 7drising
send
engaged segment only
throttle / pause
domain 0.71 ยท complaint trend rising
No dashboard visit required. The gate runs inline.
Postmaster tells you about yesterday. This answers at send time.
What actually decides placement
I have been doing email for a decade, and my companies have sent over six billion messages. SmartrMail alone had 12,000 customers. Across all of it the same four levers decide placement, in roughly this order of weight.
The four levers
ordered by how much they move placement
Lever
What it proves
Where it goes wrong
Reputation
That mail from this domain has been wanted before
A new domain has none, so volume on day one reads as an attack
Authentication
That the message is provably from you
Records exist but do not align with the visible From domain
Engagement
That humans open, reply, and do not complain
Sending to people who never asked, then reading the silence as neutral
Content
That this looks like mail, not a broadcast
Image-only mail with nothing a filter can read
Reputation and authentication are prerequisites. Engagement is what compounds.
Engagement is the one people underrate. It is a virtuous cycle: decent engagement improves inboxing, which puts more mail in front of people, which improves engagement. It runs in reverse just as reliably. The more unopened mail you leave sitting in inboxes, the worse your placement gets, which is why sending harder to a quiet list is the exact wrong move.
The 0.30% cliff
Since 2026, Gmail, Yahoo and Microsoft enforce a hard spam-complaint threshold of 0.30% for bulk senders, alongside mandatory authentication and one-click unsubscribe. Three complaints per thousand messages is the line.
Kam, who built our deliverability layer, grades every sending domain's spam rate against that threshold continuously rather than at report time, because the useful moment is before you cross it, not after. A domain drifting toward the line gets throttled automatically. Google Postmaster will also show you the number, but it shows you yesterday.
The reason the cliff matters more than any other metric is that it is the only one with a cliff. Bounce rate degrades you gradually. Complaint rate has a step function in it.
Why open rate no longer measures this
Open rate is noise now. Not as a figure of speech, as a technical fact. Apple's Mail Privacy Protection pre-loads every image, and Gmail proxies them, so an open no longer means a human opened anything. We do not track it as a KPI at all.
We replaced it with reply, activation and retained revenue, and we judge deliverability on inbox-and-primary-tab placement rather than on a contaminated number that makes a dying list look alive. If you are optimising open rate, you are optimising an artefact of image loading.
There is a related trap in content. If you send images only, Gmail, Yahoo and Outlook cannot read an image, so they see a message with no readable content and treat it accordingly. SendGrid and Mailchimp, the two largest infrastructure providers, ran a study finding that promotional mail landing in the Promotions tab is not the disaster people assume, but mail landing in spam absolutely is. Those are different problems and they need different fixes.
Fixing it in order
Deliverability problems have a diagnostic order, and doing it out of order wastes weeks. Start at authentication, because nothing downstream matters if the mail is not provably yours. Then check whether you are listed, since a live listing blocks at the MTA before any content check runs. Then look at bounce rate and list hygiene, because that is what moves reputation. Then warm the domain if it is new. Only then does content tuning pay.
Connecting a custom sending domain instead of a shared sandbox sender is the single highest-leverage move for a new sender. Chong makes the same point from the support side: verifying a domain separately so it becomes a trusted domain is what actually helps, not tweaking subject lines.
If you want to see where you stand before changing anything, run a deliverability test and measure real inbox placement rather than trusting your delivery log.
Most senders can work that order themselves in an afternoon, and the tooling page covers what to monitor while you do. If placement is still bad after all four are clean, that is the narrow case where deliverability consulting is worth paying for.
All of this applies to application mail too, and arguably matters more there: a password reset that lands in spam is a support ticket, which is the case made on transactional email. If you send through an API rather than SMTP, the email API is where the send path and these controls meet.
Email deliverability is whether your messages reach the inbox rather than the spam folder or a filtered tab. It is distinct from delivery, which only means the receiving server accepted the message.
What is a good email deliverability rate?
Inbox placement above 95% is healthy for a warmed, permission-based list. The number that gets you blocked is the complaint rate: Gmail and Yahoo enforce a 0.30% spam-complaint cliff for bulk senders.
Why do my emails go to spam?
Usually one of four things: authentication that does not align, a sending domain with no reputation yet, recipients who never asked to hear from you, or content that looks like a template rather than a message.
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