University
Sign in

Lesson 1 8 min read

How inbox placement works

Learn how mailbox providers decide between the inbox and spam, the signals that build sender reputation, and the order to debug spam placement.

Deliverability is not something software does for you. It is the long-term reputation of your domain, built from what you send and how people react to it. In this lesson you learn how mailbox providers decide where your email lands, and the order in which to look when it lands in the wrong place.

Delivered is not the same as inboxed

Two different things happen to every email.

Delivery means the receiving server accepted the message. Your provider reports it, and Broadcast shows it as delivered.

Placement means where the accepted message goes: the inbox, a tab such as Promotions, or the spam folder. Nobody reports placement to you. A broadcast can show a high delivery rate and still sit in spam for half your readers.

So “delivered” in your metrics tells you the address works. Only engagement over time tells you that people see your email.

  1. Sent Broadcast hands it to your provider
  2. Delivered The receiving server accepted it. Reported to you
  3. Placed Inbox, a tab or spam. Reported to nobody
Your reports stop at the second step. Only engagement over time hints at the third.

How a mailbox provider decides

Gmail, Outlook, Yahoo and the others each run their own filters, and none publishes the exact rules. But the inputs are well known, and they fall into three groups.

  1. Identity. Is this email really from who it says? The receiving server checks SPF, DKIM and DMARC for your domain. Mail that fails is treated with suspicion, or rejected.
  2. Reputation. What happened the last times this domain sent? Providers remember bounces, spam complaints, and whether people opened, replied to, or deleted your mail unread.
  3. Content and form. Does the message look like what people want? A working unsubscribe, a plain-text part, and normal links matter more than avoiding certain words.

Identity is a gate: fail it and the other two barely matter. Reputation carries the most weight over time. Content matters least, which is why it is the last thing to check.

  1. Identity A gate: SPF, DKIM, DMARC
  2. Reputation The most weight over time
  3. Content and form Matters least
Check them in this order. A failure on the left makes the right barely matter.

Reputation belongs to you

When you self-host Broadcast, you send from your own domain through your own provider account, such as Amazon SES or Postmark. The reputation you build is yours. It is not shared with thousands of other senders on a SaaS platform, and you keep it if you change tools.

That cuts both ways. Nobody else can damage it, and nobody else can repair it. The rest of this course is about building it and keeping it.

The rules big providers enforce

Since February 2024, Gmail and Yahoo enforce requirements on senders of more than about 5,000 messages a day to their users. They are now the baseline at any real volume:

  1. Authenticate with SPF, DKIM and DMARC. DMARC can start at p=none, but it must exist.
  2. Align your domains: the From domain must align with SPF or DKIM.
  3. Support one-click unsubscribe with a List-Unsubscribe header, and honour opt-outs within two days.
  4. Keep spam complaints under 0.3%. Treat 0.1% as your real ceiling.

Broadcast helps with each one. Domain DNS Health watches your records, unsubscribe headers are on by default, and bounces and complaints suppress addresses on their own once webhooks are set up. Lessons 2, 4 and 5 cover these in detail.

When mail goes to spam: the order to check

Work in this order. The first two resolve most cases.

  1. Authentication. Open Settings → Email → Domain DNS Health and check that SPF, DKIM and DMARC are Healthy for your sending domain. A Missing or Changed record is very likely the whole problem.
  2. Reputation. Did you start sending recently, or after a long gap? A new domain needs a warm-up. Were you fine and now are not? Look for a change: a jump in volume, a list import, or a rise in complaints.
  3. List hygiene. Check bounce and spam rates on the broadcasts list. Confirm that ESP webhooks are set up, because without them Broadcast cannot see bounces or complaints. Stop mailing people who have not engaged in months.
  4. Content. Only now: check for a plain-text part, an easy unsubscribe, few link shorteners, and not one large image instead of text.

Spam placement is often a list problem that looks like a content problem. Rewriting the subject line while you mail a stale list changes nothing.

Tools that help

The site has free tools for each layer, including an SPF record generator, a DMARC checker and an email spam checker. Use them before a big send. The deliverability guide puts all of it on one page and links each tool, and emails going to spam is the checklist for when something is already wrong.

What you learned

  • Delivered means accepted. Placement in the inbox or spam is decided after that, and nobody reports it.
  • Providers judge identity, then reputation, then content.
  • Gmail and Yahoo require authentication, alignment, one-click unsubscribe and low complaints.
  • Debug spam in order: authentication, reputation, list hygiene, then content.

Go deeper in the docs

Check what you learned

Answer all 3 questions correctly to complete this lesson.

1. Your emails start landing in spam. What do you check first?
2. Which spam complaint rate should you treat as your real ceiling?
3. Who owns your sending reputation when you self-host Broadcast?