---
title: "Design Email for Humans and Agents"
description: "Apple Mail and Gmail now generate AI summaries of emails, while agents read messages to automate tasks like logging receipts or tracking deadlines. Summarizers focus on the first 100 to 150 characters, so the main point and call to action must lead the email. Semantic HTML with h1 headings, a descriptive title, lang attributes, and role=‘presentation’ on tables creates a machine-readable outline, yet the Email Markup Consortium found 74% of emails lack an h1 and 96% lack lang. Hidden text styled to be invisible can hijack summaries, as Forcepoint demonstrated, and similar attacks have targeted Gemini in Google Workspace; such practices also trigger security flags. Testing in Gmail, Apple Mail, and Outlook, along with verifying the plain text version, confirms that both humans and agents interpret the email correctly."
---

# Design Email for Humans and Agents

[Resend](https://yomu.fyi/company/resend) · Kiersti Esparza · Oct 14, 2026

**Type:** Problem & solution

## Summary

Apple Mail and Gmail now generate AI summaries of emails, while agents read messages to automate tasks like logging receipts or tracking deadlines. Summarizers focus on the first 100 to 150 characters, so the main point and call to action must lead the email. Semantic HTML with h1 headings, a descriptive title, lang attributes, and role=‘presentation’ on tables creates a machine-readable outline, yet the Email Markup Consortium found 74% of emails lack an h1 and 96% lack lang. Hidden text styled to be invisible can hijack summaries, as Forcepoint demonstrated, and similar attacks have targeted Gemini in Google Workspace; such practices also trigger security flags. Testing in Gmail, Apple Mail, and Outlook, along with verifying the plain text version, confirms that both humans and agents interpret the email correctly.

## Context

With the introduction of AI-powered email summarization in Apple Mail (Apple Intelligence) and Gmail (Gemini), emails now have two distinct audiences: humans and automated agents. Agents also extract data and take actions from email content, such as logging receipts or tracking deadlines. This dual-audience environment requires email design to be both human-readable and machine-readable to avoid misunderstandings and incorrect actions.

## Approach / What changed

Follow six practices: 1) Place the key point and call to action in the first 100-150 characters. 2) Use live text rather than images for headlines and CTAs; alt text is often ignored by AI. 3) Use semantic HTML including h1, title, lang, dir, and role=‘presentation’ on layout tables. 4) State facts and actions in the body text; describe link destinations; include a plain text version. 5) Avoid hidden text (e.g., zero font size) and do not write instructions aimed at AI; treat all email as untrusted data when building agents. 6) Test by turning off images, asking a model to summarize, and sending test emails to Gmail, Apple Mail, and Outlook.

## Takeaways

- AI summarizers heavily weight the first 100–150 characters; the main message and call to action must appear there to avoid misrepresentation.
- Hidden text (e.g., zero font size) can hijack email summarizers and trigger security flags, as demonstrated by Forcepoint and against Gemini.
- Semantic HTML elements like h1, title, lang, and dir are often missing but critical for agent comprehension; the Email Markup Consortium found 74% of emails lack h1 and 96% lack lang.

## FAQ

### Why do AI summarizers often miss CTAs inside images?

Recent tests show agents ignore image CTAs because they parse text, not images. Alt text is also frequently ignored by AI summarizers. Putting the call to action in live text ensures it is read by both humans and agents.

### How can hidden text affect email delivery and AI interpretation?

Hidden text, such as content with zero font size or white text on white background, can be read by AI and used to hijack summaries. This practice resembles known attacks and can cause security tools to flag the email as spam or malicious.

### What is the simplest way to test if an email works for agents?

Turn off images in your email client: if the email stops making sense, the AI will struggle too. Also paste the HTML into an assistant and ask for a summary, and send test emails to Gmail, Apple Mail, and Outlook to check the preview and summary.

**Tags:** [Accessibility](https://yomu.fyi/topic/accessibility), [Agents](https://yomu.fyi/topic/agents), [AI](https://yomu.fyi/topic/ai), [Email](https://yomu.fyi/topic/email)

- Source: [Resend](https://resend.com/blog/design-email-for-humans-and-agents)
- Source URL: https://resend.com/blog/design-email-for-humans-and-agents
- Ingested by Yomu: 2026-10-14T21:00:13.867Z

[Read original post](https://resend.com/blog/design-email-for-humans-and-agents)