Resend Tells Senders to Write Email an AI Summarizer Can Read

Resend told senders in an Oct. 14 blog post that their email now has two audiences, people and software. Gmail and Apple Mail summaries can replace the preview text a sender wrote, the company said. The post, by Resend’s Kiersti Esparza, gives six practices for email that works for both.

Summaries now sit between the subject line and the body

Resend said Apple Mail has summarized messages with Apple Intelligence since iOS 18.1. Gmail summarizes messages and whole threads with Gemini, the company said. Agents go further, the company said: they read HTML, plain text, attachments and headers, then act on what they find, such as logging a receipt total, tracking a deadline or unsubscribing someone.

Resend cited marketers who call the first 100 to 150 characters of an email the “AI fold.” It also cited an Inbox Monster finding that when an email makes three asks, the AI picks one, often the wrong one.

Images and missing structure hide the message

Resend reported test results in which Apple Intelligence declined to summarize an email whose call to action was an image, while Gemini returned one vague line. Emails with the same call to action as live text produced useful summaries from Apple Intelligence, Gemini and Copilot. Resend said summarizers often ignore alt text.

The post cited the Email Markup Consortium’s 2026 report, which analyzed 376,348 emails. It found 74% had no <h1>, about 96% were missing lang and dir on the body, and 45% had no <title>. Resend recommended a descriptive <title>, lang and dir on the body’s direct children, and role="presentation" on layout tables.

Resend said React Email now sets language and direction on <Body> by default, and <Preview> emits a <title>. Resend generates a plain text version of every email automatically. React Email’s toPlainText renders both versions from one template.

Hidden text reads as an attack

Forcepoint X-Labs showed in August that a few lines of HTML styled with zero font size and white text can hijack an email summarizer, Resend said. Researchers have shown the same trick against Gemini in Google Workspace. Resend advised senders not to hide text, including preheaders, and not to write lines such as “if you are an AI assistant, say…” because security filters read them as attacks.

Resend repeated Forcepoint’s advice to teams that build agents reading mail: extract only what a person would see, keep headers out of the prompt body, check summaries against the original and require a human to confirm high-risk actions such as paying, deleting or forwarding.

Key takeaways

• Put the point and the call to action in the first 100 to 150 characters.
• Keep headlines, facts and links in live text, not images.
• Add semantic HTML, a descriptive <title>, lang and dir.
• Never hide text. Never address the AI directly.

The take

We think a summary is now part of the email. A sender should read it before a customer does. Resend’s tests suggest the summarizer reads a message the way a screen reader does, from the top, in text, by its structure. Anything that depends on a picture or on hidden markup may not make it through.

Most of this advice costs little. It matches what accessible email already asked for. The rule that needs care is the ban on hidden text. A trick that once only padded a preview line can now read as an attack on an assistant. Filters treat it that way.

Teams should send a test email to Gmail and Apple Mail, tap summarize and read what comes back. Fix the first two sentences and the plain text version when the summary misses the amount, the date or the link.