Email Design & Deliverability

The £200,000 email that nobody received

In 2021, a mid-sized online retailer prepared a Black Friday email campaign — their biggest send of the year, with a 40% discount on their entire product catalogue. 80,000 subscribers. Their email team spent two weeks perfecting the design: animated headers, large product images, a countdown timer built in HTML.

Campaign sent. Results:

• Delivery rate: 34%
• Open rate: 4.2%
• Revenue: £23,000 (vs. £180,000 projected)

The post-mortem revealed what happened: months of poor list hygiene (sending to inactive subscribers who never opened), plus the image-heavy template that triggered spam filters, had degraded their sender reputation. Gmail classified the campaign as promotional, and then as spam, for a large portion of their list.

They'd spent two weeks on email design and zero minutes on deliverability. Their inbox placement rate on the most important send of the year was roughly 34%.

(Illustrative scenario based on patterns common in email marketing. Specific figures are representative of real-world outcomes — not a verified account of a specific named company.)

What deliverability actually is

Deliverability is whether your email reaches the inbox — as opposed to the spam folder, the promotions tab, or being blocked entirely.

The inbox placement problem: Email platforms show you "delivered" rate — but delivered means the email was accepted by the receiving server, not that it reached the inbox. An email can be "delivered" and sitting in spam. Open rate is a proxy for inbox placement, with an important caveat: Apple Mail Privacy Protection (AMPP), launched in 2021, routes email content through Apple's privacy proxy, pre-loading tracking pixels before the user even opens the email. This means open-rate tracking becomes unreliable — Apple registers an "open" whether or not the user actually read the message, artificially inflating open rates for lists with many Apple Mail users. A sudden drop in open rate is still a reliable deliverability warning signal; absolute open rate numbers are less reliable than they used to be. Click rate is now the more trustworthy engagement metric.

The three pillars of deliverability

Pillar 1: Sender reputation

Email providers (Gmail, Outlook, Apple Mail) maintain a reputation score for your sending domain and IP address. This score is based on:

• Engagement signals: Open rate, click rate — high engagement = trusted sender
• Spam reports: Subscribers marking your email as spam — Google officially published these thresholds in February 2024 as part of updated bulk sender guidelines: a 0.10% complaint rate before deliverability is impacted; above 0.30% risks being blocked entirely (these thresholds apply specifically to bulk senders (5,000+ emails/day to Gmail accounts) and govern Gmail delivery specifically — other providers have their own standards). Check Google Postmaster Tools documentation for current guidance.
• Bounce rates: Hard bounces (invalid addresses) degrade reputation quickly
• Unsubscribe rate: Very high unsubscribes signal unwanted emails

How to protect sender reputation:

• Only email people who explicitly opted in
• Send consistently (sudden volume spikes look suspicious to filters)
• Remove hard bounces immediately
• Run re-engagement campaigns to clean inactive subscribers before they become spam complaints
• Include a clear, functioning unsubscribe link (make it easier to unsubscribe than to mark as spam)

Pillar 2: Technical authentication

Three technical records tell receiving email servers that you're a legitimate sender:

SPF (Sender Policy Framework): A DNS record listing the mail servers authorised to send email from your domain. Prevents spoofing.

DKIM (DomainKeys Identified Mail): A cryptographic signature added to each email proving it actually came from your domain and wasn't tampered with in transit.

DMARC (Domain-based Message Authentication, Reporting & Conformance): A policy that tells receiving servers what to do with emails that fail SPF or DKIM checks.

Most email platforms (Mailchimp, Kit, Klaviyo) provide instructions for setting up SPF and DKIM for your domain. This is a one-time technical setup (takes 15–30 minutes) that significantly improves deliverability.

Pillar 3: Content quality signals

Email filters scan content for signals of spam:

• High image-to-text ratio: Emails with more images than text look like spam (spammers use images to hide text from filters). Aim for 60% text, 40% image at most.
• Spam trigger words: "FREE!!!", "Make money fast", "Limited time offer!!!" — excessive capitalisation and exclamation marks are spam signals
• Link quality: Links to known spam domains or excessive links reduce deliverability
• Plain text version: Always include a plain text version of your email (most platforms do this automatically)

Email design: what actually works

Email design exists to serve readability and conversion — not to impress. The most effective marketing emails are often the least visually elaborate.

Design principles for marketing email:

Mobile-first always: A majority of emails are opened on mobile devices — industry tracking typically shows 40–60% or higher depending on audience (Litmus Email Client Market Share reports; figures vary by industry). A beautiful desktop email that's unreadable on a phone is a failed email. Use single-column layouts; minimum 16px font size for body text; large, easy-to-tap buttons (minimum 44px height).

The F-pattern: Research on web page reading patterns (Nielsen Norman Group) shows an F-pattern: readers skim the first line, then scan down the left edge, and occasionally read across when something catches their attention. While originally studied for web pages, this directional principle applies to email layout as well. Design to this pattern: put the most important content (headline, first paragraph, primary CTA) in the F-path.

Plain text vs. HTML design:

The typography basics:

• Body text: 16px minimum on desktop, 18px on mobile
• Line height: 1.5× font size for readability
• Limit to 2 fonts maximum (one for headers, one for body)
• High contrast: dark text on light background; never light text on white

Image best practices:

• Always include alt text on images (many email clients block images by default; alt text maintains meaning)
• Compress images (large file sizes slow loading and trigger spam filters)
• Never put critical information inside an image (subscribers who have image loading disabled miss it)
• Use images to support text, not replace it

Email template types

Plain text emails: No formatting beyond line breaks. Looks like a personal email from a colleague. Best for sequences, newsletters, and high-engagement campaigns. Highest deliverability.

Minimal HTML: Clean, simple layout with one or two colours, minimal images, branded font. Retains the "personal" feel while adding brand consistency. Most newsletters use this approach.

Full branded HTML: Company header, multiple images, product grids, prominent CTAs. Used for promotional emails, announcements, and e-commerce product launches. Higher visual impact, lower deliverability.

Responsive HTML template: A template designed to reformat automatically for different screen sizes (desktop vs. mobile vs. tablet). All modern email platforms include responsive templates — always use these rather than fixed-width designs.

Back to the £200,000 email

The £200,000 email landed in spam not because of the creative — the design team had done their job. It landed in spam because the domain authentication records weren't configured, months of sending to inactive subscribers had degraded sender reputation, and an image-heavy template pushed it past Gmail's thresholds on the most important send of the year. The email worked perfectly. The infrastructure didn't. Two weeks of design work was undone by a 30-minute technical setup that nobody had done. Deliverability isn't glamorous, but it's the foundation everything else sits on. A campaign that doesn't reach the inbox has a 0% open rate, a 0% click rate, and generates exactly £0.

Key takeaways

• Deliverability is the foundation. An email that doesn't reach the inbox generates zero results, regardless of how good the copy or design is.
• Sender reputation is built by engagement and destroyed by complaints. High open rates protect deliverability; spam complaints and large numbers of hard bounces damage it severely.
• SPF, DKIM, and DMARC are one-time technical setups with permanent deliverability impact. Set them up when you start; don't wait until you have a deliverability problem.
• Plain text often outperforms elaborate design. The most impactful emails feel personal, not designed — especially for sequences and newsletters.
• Mobile-first design is not optional. Most emails are opened on mobile — design for small screens first (Litmus Email Analytics).