Email Deliverability: The Mechanics That Decide Whether You Reach the Inbox

The campaign these numbers come from

The figures here are from one campaign we ran to fill a 300-seat event in three weeks. We use our own numbers rather than industry benchmarks because we can see every one of them. Treat them as one data point, not a law — the list matters more than any technique below.

1,578

Emails Sent

across 6 steps

48.7%

Open Rate

769 opened

9.1%

Click Rate

144 clicks

270

Leads Reached

all previously engaged

Delivery and deliverability are different problems

Delivery means the receiving server accepted the message. Deliverability means a human can find it. You can have perfect delivery and still be invisible, because the message is sitting in Promotions or in spam. The two have different causes and different fixes, so it is worth knowing which one you have before changing anything.

• Accepted but in spam: a reputation problem — look at your sending curve and authentication
• Accepted but in Promotions: a formatting problem — look at images, link count and tone
• Rejected outright: usually authentication or a bad address, and the bounce message will say which

The sending curve is the strongest signal you control

Mailbox providers score a domain on its history. A domain with no history that suddenly sends hundreds of messages in a day has one obvious explanation, and the filters all reach it. This is why a brand new domain sending its first large batch lands in spam regardless of how good the writing is.

Send from a subdomain, never your root domain

Use something like mail.yourcompany.com for outreach. Reputation attaches to the sending domain, so isolating it means a bad run damages only the subdomain. If you burn your root domain instead, your invoices, support replies and everyday business email go down with the campaign.

• Set the subdomain up before the first send, not after a problem appears
• Keep transactional mail on a different subdomain again, so a cold campaign cannot affect receipts or password resets

Authentication is table stakes now

SPF, DKIM and DMARC are no longer optional for anyone sending at volume. Google and Yahoo both began requiring DMARC for bulk senders in February 2024, and messages that fail authentication are filtered aggressively. All three records live in DNS and take minutes to add.

• SPF: lists which servers may send for your domain
• DKIM: cryptographically signs each message so the receiver can verify it was not altered
• DMARC: tells receivers what to do when SPF or DKIM fails, and sends you reports
• Check all three resolve correctly before the first send — a typo in a DNS record fails silently

Your links are scored too

Filters evaluate where a link points, not only who sent it. A public shortener is a shared domain, which means you inherit the reputation of every sender who used it before you — and some of them were not careful. This is one of the fastest ways to undo good sending practice.

• Use your own tracking domain, such as links.yourcompany.com, so it carries only your record
• Put it on the same subdomain you send from, so the link and the sender tell one consistent story
• Authenticate the tracking domain the same way you authenticate sending — receivers check both
• One link per cold email; several starts to look like bulk marketing whatever the domain

How the message is built matters for the tab you land in

Promotions is a formatting judgement rather than a reputation one, which is why it has a different fix. Gmail reads the shape of the message and decides whether it looks like a broadcast. A heavy image-to-text ratio, many links and visible template scaffolding all push you there.

Two rendering facts worth knowing before you design anything

Outlook for Windows renders HTML email with the Microsoft Word engine, and has done since 2007. Flexbox, CSS grid, max-width and background images are ignored, so a layout that looks right in a browser preview can arrive as a single collapsed column. Separately, Gmail clips messages over roughly 102KB and hides the remainder behind a View entire message link — which is usually where the call to action ends up.

• Inline CSS, table layouts and fixed pixel widths render everywhere
• Link to images rather than embedding them; base64 images are what push a small email past the clip threshold
• Preview in several clients before the first send rather than after the campaign

Give people an exit

Adding an opt-out line to cold email improves deliverability, which sounds backwards until you compare the two costs. An unsubscribe removes one contact and is invisible to your domain. A spam complaint is recorded against your sending reputation and degrades delivery for everyone else on the list. If someone wants out and you have not given them a way, they use the button that is one click away in every mail client.

• One plain sentence is enough: reply STOP and I will remove you
• A full unsubscribe footer in cold email arguably reads more like bulk mail, so keep it minimal

The part none of this fixes

Every mechanism above improves a good list. None of them rescue a bad one. The 48.7% open rate in the table came from 270 people who had already interacted with us — event signups, past attendees, people who had commented. A purchased list generates complaints from people who never asked to hear from you, and those complaints land on the domain you just spent two weeks warming.

How to build a list worth sending to ↗