What is email deliverability, and how do you improve it

Email deliverability is about whether your messages reach the inbox. A campaign report can show 98% delivered while hardly anyone opens. That happens when servers accept the mail and then file it as spam.

Delivery vs deliverability

Delivery rate counts the messages the receiving server accepted without an error. Anything that came back as a bounce is left out. Deliverability goes one step further and asks where the message ended up after it was accepted. The share that lands in the inbox itself is called the inbox placement rate.

A round-number example. You send 1,000 messages, 20 bounce, and the platform reports 98% delivered. If another 200 accepted messages went to spam, 780 reached the inbox, which is 78% of what you sent. The gap between 98 and 78 never appears in the platform report, because to the platform all 980 were delivered.

What deliverability is built from

It starts with authentication. SPF, DKIM and DMARC prove the mail is yours, and without them the big mailbox providers will not accept bulk mail, as the table below shows. On top of that sits reputation, the history of your domain and IP, which records complaints, bounces, trap hits and volume spikes. See the guide to sender reputation.

A lot depends on the list. Dead addresses bounce, spam traps hurt reputation, and people who never signed up complain. Google also says plainly not to purchase addresses from other companies, and its guidelines go on to the message itself: format HTML to standards, do not hide content with HTML and CSS, do not start subjects with "Re:" or "Fwd:" unless the message really is a reply, keep links clear and do not mix message types, such as promotions inside a receipt. Finally, providers watch how recipients react through complaints, unsubscribes and opens. The reasons why emails go to spam are covered on the domain health check page.

What Gmail, Yahoo and Outlook require

The three largest providers ask for similar things. Gmail and Outlook draw the bulk-sender line at 5,000 messages a day, while Yahoo names no number and talks about a "significant volume". Gmail and Yahoo also have rules that apply to every sender.

Details and error codes are in our write-ups of the Gmail and Yahoo requirements, and record setup is in the SPF, DKIM and DMARC guide. Beyond the mandatory part, Microsoft asks bulk senders to use a From or Reply-To address that can receive replies, to remove invalid addresses regularly, and to mail only people who consented.

How to measure deliverability

Sending platforms have no "landed in the inbox" counter, so the picture comes from several places.

• Provider dashboards. Google Postmaster Tools shows spam rate, authentication and compliance with Gmail's requirements. Microsoft has per-IP data in SNDS, and Yahoo runs a Complaint Feedback Loop.
• Test mailboxes. Send the message to your own Gmail, Outlook and Yahoo addresses and see where it lands. The pre-send QA checklist covers this. Paid seed tests do the same across dozens of mailboxes.
• Campaign metrics. A sudden drop in opens at one provider, with the same list and template, most often means mail started going to spam.

Opens come with a caveat. Apple's Mail Privacy Protection loads remote content in the background even when the recipient never opens the message. The tracking pixel fires, and that "open" says nothing about whether anyone saw the email. Compare providers against each other and lean on clicks and replies.

How to improve email deliverability

1. Set up SPF, DKIM and DMARC for every service that sends as your domain. The domain health check shows what is missing.
2. Validate the list before sending. Finding dead mailboxes and domains without mail with a validator beats finding them through bounces, especially before the first send to an old list.
3. Confirm signups. Double opt-in filters out typos, other people's addresses and bots, and validation in the signup form catches junk even earlier.
4. Act on bounces and complaints at once. A hard bounce removes the address after the first time, and a complaint unsubscribes the person.
5. Segment by activity. Send long-inactive subscribers a re-engagement campaign first and drop those who stay silent. Google suggests considering unsubscribing people who do not read your mail.
6. Make unsubscribing easy. List-Unsubscribe and List-Unsubscribe-Post headers plus a visible link in the body. Someone who cannot find the way out clicks "Report spam".
7. Grow volume gradually. New domains and IPs get warmed up. Google warns that even abruptly doubling your previous volume can trigger rate limiting.
8. Let people choose the frequency. A preference center where subscribers pick topics and cadence gives people who feel over-mailed an option other than a complaint.

The most common mistakes

The usual one is mailing a list that was never checked, or one that was bought. Another is connecting a new sending service and forgetting to add it to SPF and DKIM. Bounces pile up because an address returned 550 and still sits on the list. One message goes to the whole list, including people silent for years, with the unsubscribe link in tiny print at the very bottom, if it is there at all. Then there are emails that are a single image, which leaves a filter nothing to read, and new domains launched straight to the full list.