Deliverability overview

Deliverability is whether your mail lands in the inbox rather than the spam folder or nowhere at all. It is not luck and it is not a setting you turn on. It is the result of five things you control. This page explains what those five things are, gives you the numbers mailbox providers actually measure you against, and points you at the page that fixes each one.

The five inputs#

A mailbox provider decides where your message goes in a fraction of a second, using signals it has been collecting about you for weeks. Those signals fall into five groups.

Authentication. Can the receiving server prove the mail is really from your domain? SPF, DKIM and DMARC records answer that question. Without them, your mail is anonymous, and anonymous mail from a domain nobody recognizes is the easiest thing in the world to filter. This is the first thing to fix because it is the only one that is binary: either the records are right or they are not.

Sender reputation. Every domain you send from accumulates a history: how much you send, how steadily, how people react. A domain with six months of consistent sending to people who open the mail is trusted. A domain that sent nothing for a year and then sent 80,000 messages on a Tuesday is not. Reputation is per domain and, to a lesser degree, per sending IP.

List quality. Where the addresses came from, and how current they are. A list built from people who typed their address into your form is a different asset from a file you bought, even if both contain 50,000 rows. Old lists decay: addresses get abandoned, and abandoned addresses eventually get turned into spam traps.

Engagement. Opens, clicks, replies, and the absence of deletions without reading. Providers use engagement as the tie-breaker on everything else. Two senders with identical authentication and identical volume get different treatment if one is read and the other is ignored.

Content. The message itself: the subject line, the ratio of text to images, the links, the unsubscribe path. Content matters less than most people assume and less than the four inputs above, but a message that hides its unsubscribe link or links to a domain with a bad reputation will still be filtered.

The numbers that matter#

Providers publish requirements for anyone sending in volume. These are the ones worth memorizing.

The complaint threshold deserves a second look because the number is so small it does not feel real. One complaint per thousand delivered messages is the ceiling. On a 20,000-message send, twenty people hitting the spam button is enough to be a problem. That is why a single send to a stale list can undo months of good behavior.

Why providers care#

A mailbox provider is not judging your marketing. It is protecting its own users from a firehose. The overwhelming majority of mail attempted at any large provider is unwanted, so the default posture is suspicion, and you earn your way out of it with evidence.

Authentication is the evidence that you are who you claim to be. Consistent volume is the evidence that you are a real business with a real schedule. Engagement is the evidence that the people receiving your mail want it. Complaints are counter-evidence, which is why they weigh so much more than their small numbers suggest.

The order to fix things in#

If you do nothing else, do these four in this order.

1. Authenticate your domain. SPF, DKIM and DMARC. Nothing else you do compensates for missing this.
2. Clean the list before the first send. Run it through verification, remove the undeliverable addresses, and decide what to do with the risky ones.
3. Warm up if the domain is new. A brand new sending domain has no history, so start small and build volume over several weeks.
4. Watch bounces and complaints after every send. These are your early warning system. Act on the first bad send, not the third.

Everything after that is refinement: a tracking domain so your links carry your own name, segmentation so you mail the engaged more often than the indifferent, and regular hygiene so the list does not quietly rot.