What Is an Email Client?
An email client is the software layer that sits between your email server and the person reading your message. When a subscriber opens a campaign from a Johannesburg retailer, they are not viewing the raw email file directly. They are viewing it through a rendering engine built into their email client, which interprets the HTML and CSS code in your message and displays it on screen. Every email client has its own rendering rules, and those rules differ substantially between platforms.
Email clients fall into three broad categories. Webmail clients are browser-based and include Gmail, Yahoo Mail, and Outlook.com. Desktop clients are installed applications such as Microsoft Outlook, Apple Mail on macOS, and Mozilla Thunderbird. Mobile clients are the native mail apps on iPhone (Apple Mail), Android (Gmail app, Samsung Mail), and third-party apps like Spark or Airmail. Each category has its own strengths and rendering quirks.
The rendering engine used by an email client determines which CSS properties it supports. Apple Mail, built on WebKit, supports most modern CSS including animations and advanced layout. Gmail clips emails longer than 102KB and strips certain CSS from the head. The desktop version of Outlook uses Microsoft Word's rendering engine, which ignores many CSS rules that work perfectly everywhere else. This inconsistency is why professional email marketing always includes cross-client testing before launch.
Understanding your audience's email client mix matters for design decisions. A subscriber base dominated by corporate Outlook users requires simpler table-based layouts. A predominantly mobile Gmail audience benefits from a single-column, large-text design with generous tap targets. Most email platforms provide inbox placement reports that show the percentage of opens by client, giving you the data needed to prioritise your design and testing efforts.
Email Client In Practice
The scenario below is an illustrative example, not a Juicy Designs client result. The figures indicate the scale of effect that client-aware email design typically produces, so treat them as indicative rather than measured.
Imagine a Pretoria-based financial services company that sends a quarterly newsletter and checks the email client report in a platform like Mailchimp before its next redesign. A report of this kind might show that something in the region of 70 percent of subscribers open on mobile, split between Apple Mail (iOS) and the Gmail app, with only around 15 percent using desktop Outlook. Armed with that data, the company could shift from a complex two-column layout to a single-column, mobile-first design with 16px body text and 44px-minimum tap targets on all buttons. A design change of this kind could plausibly lift the click-through rate by around a third in the following campaign, with no change to the content itself.
The practical implication for South African businesses is straightforward: always check your email client report before redesigning a template, and use a tool like Litmus or Email on Acid to preview your HTML email across the top ten clients before sending. Fixing a broken Outlook layout after 50,000 emails have been delivered is impossible. Testing before send costs minutes. It is one of the simplest quality checks in email marketing and one of the most frequently skipped.
What an email client is
An email client is the application or programme people use to read and send email, such as Gmail (in a browser or app), Microsoft Outlook, Apple Mail, Yahoo Mail and many others, each of which renders and displays emails in its own way. For email marketers, email clients matter enormously because they do not all display an email identically: each client supports HTML and CSS differently, and often more restrictively than web browsers, so the same email can look different, or break, from one client to another. An email that appears perfect in Gmail might render with broken layout in Outlook, or differently again in Apple Mail on an iPhone, because of how each client interprets (or ignores) the email's code. This fragmentation is the central practical challenge of email design: unlike a web page rendered by a handful of broadly-standardised browsers, an email must display acceptably across a wide range of email clients and devices with inconsistent and sometimes idiosyncratic rendering. Understanding that recipients use many different email clients, each rendering differently, is why email marketers build and test emails carefully for cross-client compatibility rather than assuming an email will look the same everywhere.
Designing for email clients
Because email clients render inconsistently, designing email means building for cross-client compatibility rather than for a single ideal display. The reliable techniques reflect this: many marketers use table-based layouts and inline CSS, because numerous email clients strip out or ignore linked stylesheets and modern CSS, so more conservative, well-supported methods render more consistently across the range of clients. Testing is essential, previewing an email across the common clients and devices (Gmail, Outlook, Apple Mail on desktop and mobile, and others relevant to the audience) before sending, using preview and testing tools, catches rendering problems that would otherwise reach recipients. Some notoriously quirky clients (Outlook's rendering engine is a frequent culprit) demand particular care and sometimes client-specific code adjustments. Beyond layout, designing for email clients means accounting for images being blocked by default in some clients until the recipient loads them (so the email must still make sense with images off, using alt text and not relying solely on images), and for the heavy use of mobile email apps, making responsive, mobile-friendly design important. Knowing which clients your particular audience uses helps prioritise where to test and optimise. The overarching approach is defensive and tested: build with robust, widely-supported techniques, account for images and mobile, and verify across real clients, so the email displays acceptably for recipients whatever client they open it in, which is what designing for the fragmented email-client landscape requires.
FAQ
Why do my emails look different in Gmail versus Outlook?
Gmail and Outlook use different rendering engines to interpret HTML email code. Outlook relies on Microsoft Word's rendering engine, which has limited CSS support. Gmail strips certain styles. These differences mean a design that looks perfect in one client can break in another, making cross-client testing essential before sending campaigns.
Which email clients are most popular in South Africa?
Gmail dominates South African inboxes, followed by Apple Mail on iOS devices and Outlook in corporate environments. Mobile clients collectively account for over 60 percent of email opens in South Africa, making responsive design and mobile-first testing a priority for any local email marketing campaign.
Why do emails look different in Gmail versus Outlook?
Because email clients render HTML and CSS differently, and often more restrictively than web browsers. Gmail and Outlook interpret an email's code in their own ways, with Outlook's rendering engine being especially quirky, so the same email can display differently or break between them. This is why marketers build emails with robust techniques and test across clients before sending.