An email client is the application or service a person uses to read, send, and manage email, such as Gmail, Apple Mail, or Outlook.
Why the Same Email Can Look Different to Different People
What is an email client responsible for that a lot of senders don’t fully appreciate until something breaks: each one renders HTML and CSS slightly differently, some strip certain styling entirely, some clip long emails, some render images by default while others block them until the recipient opts in.
An email that looks perfect in one client can display with broken spacing, missing images, or a completely different layout in another, not because anything was sent incorrectly, but because each client interprets the same code through its own rendering engine.
Where the Real Concentration Sits
Email client examples span a long list, but the actual market is far more concentrated than most senders assume:
- Apple Mail holds above 55 percent of the global email client market.
- Gmail trails at roughly 30 percent.
- Together, these two clients cover close to 87 percent of all email opens.
This concentration matters practically: testing thoroughly on just these two clients covers the overwhelming majority of a list, while chasing perfect rendering across dozens of smaller, rarely-used clients offers diminishing returns for the effort involved.
Email client market share also shifts gradually over time as new versions roll out privacy or rendering changes, which is why senders who last checked their client breakdown a year or two ago may be optimizing for an outdated picture of where their audience actually reads email.
The Privacy Feature That Complicates Everything
Apple’s Mail Privacy Protection, introduced across Apple Mail, pre-fetches images the moment an email lands rather than when a recipient actually opens it, which means a large share of what platforms report as “opens” from Apple Mail users may not reflect genuine engagement at all.
Some datasets attribute more than half of all tracked opens industry-wide to this automatic pre-fetching behavior. This doesn’t make open rate useless as a metric, but it does mean a sender relying heavily on open rate to judge performance, particularly for a list skewed toward Apple Mail users, should weight click-through and conversion data more heavily, since those signals require genuine recipient action in a way a pre-fetched open does not.
Designing Around Rendering Differences
Email client rendering inconsistency is the reason responsive, table-based, or well-tested HTML matters more in email than it does on the open web, where modern browsers largely converged on shared standards years ago.
Email hasn’t had that same convergence, Outlook in particular still renders using a Word-based engine rather than a standard browser engine, which produces layout quirks that don’t show up anywhere else.
Testing across the top clients before every send, rather than assuming a design that looks right in one client looks right everywhere, remains a genuinely necessary step rather than an optional one.
Related terms:
Adflipr’s drag-and-drop email builder generates code tested to render consistently across major email clients, reducing the guesswork of manual coding.
