Everything that decides whether your email reaches the inbox — authentication, infrastructure, list hygiene, content, engagement and monitoring — in one actionable checklist.
These two words get used interchangeably and they mean very different things.
Delivery is whether the receiving server accepted your message. A 250 OK means delivered. Deliverability is whether that accepted message landed in the inbox rather than the spam folder or a silent black hole. You can have 99.8 percent delivery and terrible deliverability at the same time, and nothing in your SMTP logs will tell you.
This checklist covers every lever that moves the second number. Work through it in order; the earlier sections are prerequisites for the later ones.
-all.d= tag.p=none to p=reject once reports are clean.v=spf1 -all and p=reject.Gmail, Yahoo and Outlook all enforce authentication for bulk senders. Failing any of the first four items caps your deliverability no matter how good your content is.
The subdomain split is the highest-leverage infrastructure decision on this list. Send receipts from mail.yourdomain.com and campaigns from news.yourdomain.com. A bad marketing week then cannot push password resets into spam.
Mailbox providers score both your domain reputation and your IP reputation. Domain reputation matters more and follows you across providers; IP reputation resets if you move.
Typical warm-up curve: 100 on day one, 500 by day four, 2,000 by day eight, 10,000 by day twelve, 50,000 by day twenty, full volume after day twenty-five.
Poor list quality causes more spam-foldering than any other single factor.
Two types of spam trap matter. Pristine traps are addresses never used by a human, published to catch scrapers — hitting one implies you did not collect the address legitimately. Recycled traps are abandoned mailboxes that a provider reactivates as traps — hitting one implies you do not remove inactive subscribers. Sunsetting protects you from the second kind.
Target thresholds: hard bounce rate below 2 percent, complaint rate below 0.1 percent (Gmail's hard limit is 0.3 percent), unsubscribe rate below 0.5 percent.
noreply@ — it discourages replies, and replies are a strong positive engagement signal.Avoid URL shorteners: they are heavily abused and filters treat them with suspicion. Host your links on a subdomain of your sending domain, and make sure that domain is not itself on a blocklist.
Modern filters use machine learning over engagement, not a keyword blacklist, so the old "never write FREE" advice is outdated. What still hurts:
What helps: subject lines between 30 and 50 characters, a useful preheader, personalisation that is genuinely relevant, and a single obvious call to action.
Engagement is the strongest modern deliverability signal. Providers watch opens, clicks, replies, forwards, marking as not-spam, and moving out of spam — and equally, deletions without opening, marking as spam, and ignoring.
Compliance and deliverability reinforce each other. Every requirement above also reduces complaints.
When deliverability drops, work the diagnosis in this order:
Do not respond to a reputation problem by sending more mail to more people. That is the reflex, and it always makes things worse.
Ten minutes a week prevents the multi-day outage that follows from noticing a problem a month late. Before any configuration change reaches production, confirm the transport layer still works end to end with SMTPTester — authentication, TLS negotiation, and relay in one pass.
Modern spam filtering is not a keyword scanner. It is a machine-learning system weighing hundreds of signals, and the weights differ by provider.
Gmail leans heavily on individual engagement history. If a specific recipient regularly opens your mail, it reaches their inbox even when your aggregate reputation is mediocre. If they never open it, it eventually stops reaching them regardless of how clean your setup is. Gmail also applies tabbed categorisation — Promotions is not the spam folder, and landing there is normal for marketing mail.
Microsoft weights IP reputation more heavily than Gmail and is comparatively strict about sudden volume changes. It also runs its own filtering stack for both consumer Outlook.com and corporate Microsoft 365 tenants, with different thresholds.
Yahoo and AOL share infrastructure and are unusually sensitive to complaint rates.
Corporate gateways such as Proofpoint, Mimecast and Barracuda sit in front of many business mailboxes and apply their own policies, often stricter than the mailbox provider's. Attachment types and link reputation matter more here.
The practical consequence: measure deliverability per provider, not in aggregate. A single number hides the story.
Beyond obvious spam signals, several technically-correct choices reduce placement:
The hardest part of deliverability is that inbox placement is invisible from your side. You know a message was accepted; you do not know where it landed. Three techniques help:
Seed lists. Send campaigns to a set of test mailboxes across the major providers and record where each one lands. Cheap and directional, though seed accounts have no engagement history so results skew pessimistic.
Panel data. Commercial services measure placement across a panel of real mailboxes. More accurate, and priced accordingly.
Engagement proxies. A drop in open rate for a specific provider, with no change in content or list, is strong evidence of a placement problem at that provider. This costs nothing and is often the earliest signal you get.
Deliverability degrades gradually and then fails suddenly. A calendar prevents that.
Daily: check bounce and complaint rates against thresholds; process the suppression queue.
Weekly: review Google Postmaster Tools reputation; skim DMARC reports for unknown sources; check blocklists.
Monthly: run a seed test; review engagement segments; sunset newly inactive subscribers; verify authentication records still resolve.
Quarterly: audit every third-party sender; rotate DKIM keys and SMTP credentials; review consent records; re-verify any list that has not been mailed recently.
None of these take long. The teams that keep excellent inbox rates are not doing anything exotic — they are doing ordinary things consistently, which is exactly why their reputation stays stable while everyone else's oscillates.
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Open the tool →A complete guide to testing SMTP servers — online testing, telnet and openssl commands, TLS verification, authentication checks and a repeatable diagnostic workflow.
DeliverabilityHow bounces work, the difference between hard and soft bounces, how to parse DSN messages, build a suppression list, and keep bounce rates under control.