Core Concepts

Email clients

An email client is a combination of platform and rendering engine. Emailens supports 21 clients across webmail, desktop, and mobile categories.

Each client has different CSS support. For example:

• Gmail keeps a <style> block in the <head> (up to 16KB), but its apps drop it for non-Google accounts, so critical styles must be inline
• Outlook Classic uses the Microsoft Word rendering engine, which doesn't support border-radius, flexbox, or grid. The new Outlook for Windows uses a modern web engine with much better CSS support.
• Apple Mail uses WebKit and supports most modern CSS

Transforms

A transform takes your HTML and rewrites it for a specific email client. This includes:

• Inlining styles (for clients that strip <style> blocks)
• Removing unsupported properties
• Adding client-specific fallbacks
• Converting units and values

The result is client-specific HTML that replicates how the email will render. Previews are rendered in Chromium and may differ slightly from native client rendering.

Warnings

A CSS warning flags a compatibility issue. Each warning has:

• severity: error (won't render), warning (degraded), or info (minor)
• client: which client is affected
• property: the CSS property causing the issue
• message: human-readable description
• suggestion: how to fix it
• fix: before/after code snippet (when available)

Compatibility score

Each client gets a 0-100 score based on the warnings found:

• Errors have the highest impact
• Warnings reduce the score moderately
• Info issues have minimal impact

The overall score is the worst client's score, not an average. An email is only as good as the client it breaks in: twenty clients at 100 and Outlook at 60 scores 60.

Dark mode

Emailens previews how email clients render your email in dark mode. Clients that support dark mode may:

• Invert background and text colors
• Override explicit colors with system defaults
• Apply prefers-color-scheme: dark media queries

Dark mode preview produces a separate HTML variant and additional warnings specific to dark mode rendering.

Input formats

Emailens accepts four input formats:

Set the format field in your API request to use JSX, MJML, or Maizzle. The response includes the compiledHtml field with the intermediate HTML.

Framework-aware fixes

When you specify a format, CSS warnings include framework-specific fix snippets instead of generic HTML. For example, a display:flex warning in Outlook returns:

• HTML: a <table> with <!--[if mso]> conditional comments
• React Email (jsx): <Row> and <Column> components from @react-email/components
• MJML: <mj-section> and <mj-column> elements
• Maizzle: Tailwind utility classes with MSO conditional comments

This means fix suggestions are always in your source language , you can paste them directly into your code.