An email QR code builds a mailto link with your address, subject, and a starter message. One scan and the visitor is looking at a draft - which is much more likely to be sent than a blank compose window.
Use it on booth banners, invoice PDFs, and “talk to us” cards. Keep the subject specific (“Quote request - scanned from flyer”) so you can filter those messages in Gmail later.
Tips that keep replies useful
Pre-fill a couple of questions in the body so people know what to tell you. Avoid huge signatures in the encoded body; they bloat the code. If you change inboxes often, point the QR code at a contact page instead and use a dynamic URL code.