To take the tracking logic further, I think you'd do `businesscard-<date for that particular print run>@example.com`, `<name of conference>-<year>@example.com`,

To take the tracking logic further, I think you'd do `businesscard-<date for that particular print run>@example.com`, `<name of conference>-<year>@example.com`, `<name of mailing list>@example.com` etc. Maybe come up with something snazzy yet still unique-ish for the presentation slides. In practice, I suspect the intention is mostly to use catch-all addresses for situations where the email address is a key in a database and like a login and stuff and not an entry in an individual's contacts list.