Deliverability is not the same as delivery
An email can be “delivered” in the sense that the receiving server accepted it, and still never be seen. It may sit in the spam or junk folder, or in a secondary tab. Deliverability is the wider question of whether a legitimate message reaches the place where the reader will actually notice it.
No sender controls this completely. Mailbox providers such as Gmail, Outlook and Yahoo make their own decisions, and they do not publish the full rules. What you can do is understand the signals they are known to consider and build habits that keep those signals healthy. This article is a map of those signals; the other articles in this cluster go deeper on each one.
The main things that decide placement
Most explanations group the factors into four areas.
Identity. Providers want to know that a message really comes from the domain it claims to come from. This is handled by technical records called SPF, DKIM and DMARC, together known as email authentication. Without them, your mail looks similar to forged mail.
Reputation. Providers keep a running view of each sending domain and address, built from how recipients have reacted in the past. Complaints, bounces and low engagement tend to count against you. Steady, wanted mail tends to count for you.
List quality. Who you send to matters as much as what you send. Old, purchased or mistyped addresses produce bounces and complaints, which damage reputation for every message afterwards.
Content and behaviour. Subject lines, formatting, links and sending patterns can all trigger filtering, though content is usually less decisive than the first three.
Start with permission
The most durable protection is sending only to people who asked to hear from you. Marketing consent rules differ by country and change over time, and some regions require specific forms of marketing permission before you send promotional email. Check the rules that apply to you; this article is general education, not legal advice.
Permission is also practical. A person who chose to subscribe is less likely to report you as spam and more likely to open. The choice between single opt-in and confirming each signup is covered in the article on double opt-in, and it is worth deciding on purpose rather than by default.
Set up authentication before you scale
If you send from your own domain, check that SPF and DKIM are set up through your email service, and consider adding a DMARC record. Your sending service usually provides step-by-step instructions, and the exact steps differ between services, so follow their current documentation.
Authentication does not guarantee inbox placement. Think of it as the entry requirement: without it, other good habits may count for less.
Build reputation gradually
A new domain or address has no history, and providers tend to be cautious with unknown senders. Sending a large campaign on day one is a common way to trigger filtering. A gradual increase in volume, starting with your most engaged contacts, gives providers time to see that your mail is wanted. The process is usually called domain warm-up.
Reputation is also affected by what happens after you hit send. If many recipients ignore your messages, delete them unread or mark them as spam, providers often read that as a sign the mail is unwanted. Review your email metrics regularly, paying attention to complaints and bounces as well as opens and clicks. Note that open numbers can be unreliable because of privacy features in some mail apps, so treat them as a rough guide.
Keep the list healthy
A routine of list hygiene keeps problems small. Remove addresses that bounce permanently, treat repeated temporary failures with care, and decide how long a subscriber can stay silent before you try to win them back or let them go. A simple win-back campaign followed by removal is far better for your standing than mailing an ever-growing group of people who never respond.
As an illustration only: if you send to 5,000 addresses and 400 have not interacted in a long time, that is 8 percent of the list. Those 400 may be costing you more in reputation than they bring in value. The right threshold depends on your business, so choose one and apply it consistently.
Make leaving easy
It sounds backwards, but a clear unsubscribe link protects you. A person who cannot find the exit will often click the spam button instead, and complaints are among the most damaging signals. Many laws also require a working unsubscribe mechanism. Honour requests promptly, and see the notes on the unsubscribe process in this cluster for details.
Check content and consistency
Content rarely causes problems on its own, but some patterns draw attention: misleading subject lines, very large images with little text, broken or mismatched links, and messages that look very different from what recipients expect. Write clearly, match the promise in the subject line, include your real business name and address where required, and keep a recognisable sender name.
Consistency helps too. Sudden large changes in volume, sending domain or style can look unusual to a filter. If you plan a big send, such as a seasonal promotion, increase volume in steps where you can.
A simple routine
- Confirm authentication is in place after any change to your email service or domain.
- Send to people who opted in, and keep records of how they signed up.
- Remove hard bounces promptly.
- Review complaints, bounces and engagement after each campaign.
- Run a re-engagement sequence for quiet subscribers on a schedule, then remove those who do not respond.
- Keep the unsubscribe link visible and working.
- Test messages before sending and watch for placement problems, then adjust.
Outcomes vary. Even careful senders sometimes see messages filtered, and provider rules change, so check the current documentation of your email service and the mailbox providers you care about most.