A receipt is one of the few emails people still have a reason to open.
That makes it valuable real estate.
Order confirmations, password resets, shipping notices, billing statements, reservation confirmations, and account alerts are expected because they help complete or document something the recipient already did.
The Federal Trade Commission draws a legal distinction between commercial email and transactional or relationship email under CAN-SPAM. Its guidance says transactional messages include material that confirms a transaction, gives account or warranty information, or delivers a product or service the recipient already agreed to receive. See the FTC’s CAN-SPAM compliance guide.
The interesting part begins when the necessary message also becomes an advertising container.
The trusted message gets a second job
A store sends a shipping confirmation.
At the bottom are five product recommendations.
A bank sends an account notice followed by a credit-card offer. A reservation confirmation includes a large block of unrelated upgrades. A password-reset email tries to sell a premium plan.
The advertisement may be perfectly lawful. The problem is not automatically illegality.
The problem is that the user cannot simply ignore the entire message class without risking missing something important.
The FTC recognizes this mixed-content problem explicitly. When a message contains both transactional and commercial material, the primary purpose matters. If the subject line or the placement and prominence of the commercial content makes the message primarily promotional, CAN-SPAM treats it as commercial.
That rule reflects ordinary user expectations surprisingly well.
A receipt with a small related offer still behaves mostly like a receipt.
A giant advertisement with a receipt hiding underneath behaves like advertising that found a better disguise.
Essential channels should remain legible
Gmail’s current bulk-sender rules also distinguish promotional messages from transactional ones. One-click unsubscribe is required for marketing and promotional mail, while messages such as password resets and reservation confirmations are treated differently. See Gmail’s sender-guidelines FAQ.
That distinction becomes less useful when companies deliberately blur the two categories.
Users then have to inspect every service message to decide whether it contains the thing they need, another offer, or both.
Spam Empires are not only about strangers blasting millions of messages.
They are also about the industrial search for any communication surface that still reliably gets opened.
A receipt earns attention because it is necessary.
The temptation is to spend that attention twice.