An email QR code encodes a mailto: link rather than a web address. When scanned, the phone opens its default mail app with the recipient, and optionally the subject and body, already filled in. That removes the friction of copying an address off a poster or a business card, which is why it works well for support desks, feedback cards, warranty registrations, conference badges and 'report a problem' labels on equipment.
The mailto: format is simple and has existed for decades. You can append a subject and a body using query parameters, with text URL-encoded so spaces and special characters survive. Keep the pre-filled text short: long payloads produce dense codes that are slow to scan on older phones.
One caveat worth knowing: mailto links rely on the device having a configured mail app. A phone with no mail account set up will do nothing, or prompt to configure one. For critical customer-facing flows, consider a link to a web form on your own site instead - and if you use an email code, put the plain address in small text next to it as a fallback.