border

Client-by-client behaviour for border

Fully supports border (19): Gmail, Gmail Android, Gmail iOS, Outlook 365, Outlook iOS, Outlook Android, Outlook for Mac, Apple Mail, Apple Mail iOS, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, Samsung Mail, Thunderbird, HEY Mail, Proton Mail, AOL Mail, Fastmail, Superhuman.

Partial support (2): Outlook (New), Outlook Classic. Expect rendering quirks unique to each engine. Partial support typically means a subset of values, an ignored shorthand, or sanitizer-specific rewrites.

When to use border in email

• Drawing dividers between sections in a digest email by setting border-bottom on a <td>.
• Outlining promotional cards with a colored stroke that ties into brand identity.
• Highlighting CTA buttons with a contrasting border that survives image-blocking.

Rendering behaviour and edge cases

• Outlook on Windows occasionally renders an unstyled <table> with default 1px borders , explicitly set border="0" cellspacing="0" cellpadding="0" on every layout table.
• Apple Mail and Gmail support all border styles (dashed, dotted, double) but render them at slightly different pixel widths.
• border on inline elements can be stripped by Yahoo's sanitizer , use a wrapping <span> with display:inline-block when targeting Yahoo.

Recommended fallback strategy

Always reset table defaults with border="0" cellspacing="0" cellpadding="0". Apply border on <td> rather than <table> for consistent rendering. For decorative borders, prefer a colored cell background over a thin border, since cell color is more reliably preserved than 1px lines.

Fixes & Workarounds