Email deliverability: authenticate, send to people who engage, keep complaints low.

An email can be accepted and still never seen. The receiving server says yes, and the message goes to spam, to a promotions tab or to a folder nobody opens. Your delivery rate looks perfect while your readers shrink.

Mailbox providers do not tell you where an accepted email went. You read it from the pattern. Look at bounces, blocks, opens, clicks, unsubscribes and complaints together, split by provider: Gmail, Yahoo and AOL, Microsoft. A drop at one provider and not the others points at that provider's view of you, not at your content.

Context matters more than any single number. A low complaint rate looks healthy. If opens fall at the same time, it can mean the opposite: your mail now lands in spam, where people neither read it nor report it. Since Apple Mail Privacy Protection, opens are also inflated for Apple Mail users, so clicks are the more honest engagement signal.

Authentication

SPF lists the servers allowed to send for your domain. DKIM signs each message with a key published in your DNS, so the receiver can check nothing was changed and that your domain sent it. DMARC tells the receiver what to do when a message fails those checks, and asks for the domain in the From address to match the one SPF or DKIM verified. Without them, or with them set up wrong, your mail is rejected or filtered. The DMARC overview explains how the three fit.

Sender reputation

Providers keep a reputation for your sending domain and IP address, built from history: how much you send, how steadily, and how recipients react. Sudden jumps in volume or erratic schedules look like a compromised account. A new domain or IP needs a gradual warm-up. A consistent From name and a recognisable domain help people trust the email before they open it.

Engagement

Opens, clicks and replies tell the provider people want your mail. Deleting unread, ignoring it for months or moving it to spam tells it the opposite. This is the signal you control most, and it is decided by relevance.

Complaints, bounces and list quality

Every spam report counts against you. Hard bounces, addresses that do not exist, show a list that was bought, scraped or left to rot. Old addresses can be recycled as spam traps, which exist only to catch senders who never clean their lists.

Content

Content matters less than it once did, but it still counts. Misleading subject lines, shouting capitals, strings of exclamation marks and an email that is one big image with a dozen links all look like spam to a filter and to a person.

Start with the providers' own tools. Google Postmaster Tools shows your domain's spam rate as Gmail measures it, plus authentication results. Yahoo's Sender Hub and Microsoft's Smart Network Data Services do the same for their users. These are the only numbers that come from the receiving side.

Then watch your own metrics per provider and over time: hard bounces, blocks, clicks, unsubscribes and complaints. Compare each campaign with your own recent average rather than with an industry figure. Industry benchmarks for deliverability rates are widely quoted, but they come from vendors who rarely publish the method, so treat them as rough.

Our own data gives one useful baseline for online stores. Across the online stores on Releva, April 2025 to August 2026, an email at the median store in Bulgaria got 0.92% clicks and 0.06% unsubscribes (email marketing statistics). If you sit far below that, look at relevance and list quality before you look at subject lines.

1. Fix authentication first

Publish SPF and DKIM for the domain you send from, then a DMARC record. Start with p=none and read the aggregate reports to find every service sending in your name: the shop, the helpdesk, the invoicing tool. Once they all pass, move to quarantine and then reject. That also stops others from spoofing your domain.

2. Make leaving easy

Put a one-click unsubscribe header in every marketing email and a visible unsubscribe link in the footer, and act on it within two days. A reader who cannot find the exit presses the spam button instead, and that costs far more.

3. Send what each person wants

Relevance is the strongest deliverability lever, because it drives the engagement providers measure. Segment by what people bought, viewed and clicked, not only by signup date. A fashion store that shows dresses to the people who buy dresses gets clicks, and the clicks keep the next email out of spam. Our data shows the cost of the opposite: campaigns to lists under 15K clicked at 3.8 times the rate of campaigns to lists over 100K, the same period and stores as above.

4. Clean the list on a schedule

Stop sending to addresses that hard-bounce, at once. Find the people who have not clicked for months, send them a short win-back series, and stop mailing those who still do not respond. Use double opt-in at signup so typos, fake addresses and spam traps never enter the list. A smaller list that reads your mail is worth more than a large one that ignores it.

5. Keep the content plain

Write a short subject line that says what is inside. Avoid fake "Re:" and "Fwd:" prefixes, capitals and runs of punctuation. Put the main point at the top, balance images with text, and keep links few and clearly tappable on a phone.

6. Send steadily, and test

Ramp volume gradually on a new domain or IP and avoid sudden spikes before a sale. Test one thing at a time, subject line, content or send time, and judge it by clicks and orders, not by opens.