Understanding Email Deliverability

Deliverability is whether your email reaches the inbox, not just the receiving server. This article explains how that differs from “delivery”, what decides where your email lands, and why the answer changes over time. Everything else in this manual builds on these ideas.

Delivery is not deliverability

Delivery means the receiving server accepted your email. It answers your sending server with a short “OK” code (250), and your email service counts the message as delivered.

Deliverability means the email reached the inbox, where the reader can see it. An accepted email can still go to the spam folder. Some providers even accept an email and then quietly delete it.

Think of delivery as getting through the front door. Deliverability is being invited to sit at the table.

Every mailbox provider (the company that hosts the reader’s inbox, such as Gmail, Outlook.com, Yahoo or iCloud Mail) runs its own filters. You can’t see the score they give your email. It changes over time and can differ for each reader.

Where your email can end up

Once the receiving server accepts your email, one of three things happens.

The inbox

The email appears in the main inbox or in a tab such as Gmail’s Promotions or Updates. The reader can find it without searching. This is the goal.

The spam folder

The server accepted the email but filed it as spam (Outlook calls it “Junk”). It counts as delivered, but few people look in their spam folder, so in practice the email is lost.

Common reasons:

• SPF, DKIM or DMARC is missing or failing (these prove the email really comes from your domain).
• Your domain or sending server has a poor reputation.
• Many readers marked your earlier emails as spam.
• The content looks like known spam, such as misleading subjects or disguised links.
• This reader has marked similar emails as spam before.
• You suddenly sent far more email than usual.

Silently dropped

The provider accepts the email, then deletes it without telling anyone. You get no bounce (an “undeliverable” message). The reader never sees it. Providers do this with mail they consider risky or abusive.

You can’t spot silent drops in your sending reports. You only find them by sending test emails to real inboxes and checking where they land. Mailwel’s test inbox does this for you.

So a 99% delivery rate tells you little if half of those emails sit in spam. The number that matters is your inbox placement rate: the share of your emails that reach the inbox.

The three kinds of trust

Providers decide where your email goes by asking how much they trust you. That trust comes from three places.

Technical trust

These are checks on your setup, run before anything else:

• SPF (a DNS record that lists the servers allowed to send email for your domain).
• DKIM (a digital signature on each email that proves your domain sent it and nobody changed it).
• DMARC (a DNS record that tells providers what to do when SPF and DKIM don’t match your From address).
• Reverse DNS (a record that turns your server’s IP address back into a name, like mail.shop.example). That name should point back to the same IP address.
• TLS (encryption of the connection between two mail servers while the email is passed over).

Failing one of these doesn’t always mean spam, but it makes the inbox much less likely. Passing them doesn’t guarantee the inbox either. They are the minimum.

Behaviour trust

Providers watch what readers do with your emails.

• Good signs: opening, clicking, replying, forwarding, moving your email out of spam, adding you to contacts.
• Bad signs: marking as spam, deleting without reading, ignoring you for months.

An unsubscribe is a mild sign of lost interest. It is far better for you than a spam report.

For large providers like Gmail, what readers do counts for a lot. Perfect setup with readers who ignore you can still end in spam. Readers who love your emails can make up for small technical problems.

History trust

Your past sending builds a long-term reputation:

• Steady sending builds trust. Sudden spikes look suspicious.
• Spam complaints stick. A short burst of complaints can take weeks to recover from.
• New domains have no history, so providers are careful with them.
• Shared IP addresses (used by many senders at once) carry everyone’s reputation. A dedicated IP carries only yours.

The three work together. Good setup with poor engagement still lands in spam. Great engagement from a blocklisted server is still blocked at the door.

Inbox placement is a probability

Placement is not a fixed yes or no. Each email gets a fresh decision, and the odds change with every send.

How the decision is made

A provider checks the sending server, your authentication, the content, your reputation, and how likely this reader is to want the email. The combined score decides the folder.

So the same email can reach one Gmail user’s inbox and another’s spam folder. If one reader always opens your emails, you reach their inbox. If another always ignores you, you drift toward their spam.

How you send matters

Providers compare today’s sending with your usual pattern. Slow, steady growth looks responsible. A sudden jump in volume brings slowdowns and extra checks. Even a good change, like a burst of new subscribers, can lower your inbox rate for a while.

Every send feeds the next

Readers react to each email. Their reactions update your reputation, and your reputation shapes how the next email is scored. A good campaign makes the next one easier. A bad one makes it harder. Nobody gets a permanent pass to the inbox. Signal flow explains this loop in detail.

Why it matters to every sender

Deliverability affects you whether you send 100 emails a day or 10 million.

• Order and account emails: a password reset or login code in spam means an angry customer and a support ticket.
• Newsletters and marketing: an email in spam earns nothing, however good it is.
• Apps and software services: welcome emails and alerts keep people using your product. When they go missing, users quietly leave.
• Cold outreach (emails to people who never signed up): a few complaints can damage your domain within days.

If you send 5,000 or more emails a day to Gmail or Yahoo users, they set firm rules. You need SPF, DKIM and DMARC, a one-click unsubscribe link, and a spam complaint rate below 0.3%. Microsoft (Outlook.com) has required SPF, DKIM and DMARC from bulk senders since May 2025.

Problems grow quietly. Ignored bounces and complaints lower your reputation a little with each send. By the time most people notice, the damage has been building for weeks. Regular checks catch it early.

What to do next

• Send a test to the Mailwel test inbox to see your SPF, DKIM and DMARC results and how your sending is set up.
• Read Signal flow to see how signals move between you, providers and readers.
• Read The email value chain to meet everyone who affects your email.
• Read The signal game to learn which signals count most.
• Then move on to authentication, reputation and the playbooks for fixing specific problems.