Which email clients are most popular?

As of 2024, Apple Mail and Gmail dominate, together accounting for 60-70% of opens. Outlook remains important for B2B audiences. Check your own analytics for your audience's specific client distribution.

Why does my email look different in Outlook?

Outlook uses Microsoft Word's rendering engine, which has limited HTML/CSS support. It ignores many modern CSS properties, requires table-based layouts, and has VML fallbacks for some features. Test specifically for Outlook.

Should I optimize for dark mode?

Yes, as dark mode adoption grows. Test how your emails appear in dark mode (many clients auto-invert colors). Use transparent PNGs for logos, avoid all-white backgrounds, and consider adding dark mode specific CSS where supported.

Back to Glossary

Tools & Platforms

Email Client

Software or app used to read, compose, and manage email messages.

Definition

An email client is a software application or web service that allows users to send, receive, and manage their email. Email clients can be desktop applications (Outlook, Apple Mail, Thunderbird), web-based services (Gmail, Yahoo Mail, Outlook.com), or mobile apps. Each client renders HTML emails differently, which affects how your emails display.

Why It Matters

Email clients vary significantly in how they display HTML emails. What looks perfect in Gmail might break in Outlook. Understanding the email client landscape helps you design emails that work everywhere and prioritize testing for your audience's most-used clients.

How It Works

Email clients connect to mail servers using protocols like IMAP or POP3 to retrieve messages and SMTP to send them. They render HTML and CSS to display formatted emails, but each client has its own rendering engine with different capabilities and quirks. Some clients block images by default or strip certain CSS properties.

Best Practices

1Test emails in the clients your audience uses most
2Use email-safe HTML and CSS (table-based layouts, inline styles)
3Always include a plain-text version as fallback
4Consider how images display when blocked
5Monitor email client share in your analytics

Email Client Rendering Table

Client family	Common rendering issue	Design safeguard
Classic Outlook for Windows	Limited CSS support and Word-based rendering	Use table layouts and tested fallbacks
New Outlook for Windows	WebView2-based rendering with its own email constraints	Test separately from classic Outlook
Gmail	CSS and clipping limits for large emails	Keep code lean and inline critical styles
Apple Mail	Strong rendering but privacy-protected opens	Avoid relying on opens alone for engagement
Mobile clients	Narrow width and thumb-based interaction	Use large buttons and single-column layouts

Email Client Testing Priority Table

Audience	Clients to prioritize	Why
B2B SaaS	Outlook, Gmail, Apple Mail	Work inboxes often include Outlook and Google Workspace
Consumer ecommerce	Gmail, Apple Mail, Yahoo, mobile apps	Personal inboxes skew mobile and webmail
Enterprise buyers	Outlook desktop and Outlook web	Corporate environments may lag modern CSS support
Developer audience	Gmail and Apple Mail	Tech audiences often use modern clients but still need mobile testing

Email Client Feature Support Table

Feature	Support risk	Safer fallback
Background images	Poor in some Outlook versions	Solid background color and VML fallback
Custom fonts	Inconsistent across clients	Web-safe font stack
Dark mode colors	Auto-inversion can alter branding	Test transparent assets and contrast
Interactive elements	Limited or unsupported in many clients	Link to hosted interactive experience