Someone on your team still writes the receipt. Something else reads it first. Gmail summarizes the thread. Outlook offers a gist. A phone assistant reads the first two sentences aloud. An agent in the customer’s stack decides whether to file, reply, or ignore.
If the machine preview is wrong, the person may never open the mail. That is a product problem, not a copywriting hobby.
What the machine sees
Summarizers are blunt. They weight the subject, the first text paragraph, and anything that looks like a fact: amounts, dates, URLs, names. They under-weight hero images, button labels inside pictures, and the third column of a table. They will happily invent a tone you did not write if the body is vague.
Plain text is not optional. HTML-only mail is a bet that every reader renders your layout. Assistants often do not.
Write for the gist, then the human
- Subject that states the event: “Receipt for invoice 1842 — $240.00” beats “Thanks for being a customer.”
- First sentence that repeats the fact, including the action if there is one.
- One link, in text, that works if the buttons die.
- No essential copy that exists only in an image.
Preheaders still matter, but they are no longer the only preview. The summary is. If your preheader says “View in browser” and the body opens with a logo, the gist will be empty.
Agents on the other side
Some of your recipients are software. They will parse for invoice numbers, reset URLs, and calendar times. Give them a stable shape: the same phrase for the same event, a single canonical URL, a text part that matches the HTML. Do not rotate synonyms for “pay now” across templates unless you enjoy breaking someone else’s workflow.
This is the same discipline as writing for accessibility. A screen reader and a summarizer fail in similar places.
Check the preview before you ship
Paste the plain-text part into a notes app and read only that. If you cannot tell what happened, neither can the model. Then send a real message to Gmail and look at the thread summary, not just the inbox row.
For the template-level version of this, use how to write product email that Gmail can summarize. For the channel itself, what transactional email is is still the distinction most teams blur. ArawaMail is the sending side: authenticated product mail and a mailbox when a human has to answer the summary that came back.