When you book flights through Duffel, we require you to provide an email address and phone number for each of the passengers.
Airlines use this information primarily to send customers "operational notifications" related to their flights - for example if there is a schedule change, or a flight is disrupted by bad weather.
A small number of airlines - mostly "low cost carriers" - will also send the passenger a booking confirmation with their itinerary, and sometimes the price paid for their booking. If you'd like to know what airlines do this and which don't, just get in touch.
Airline industry rules explicitly say that airlines must not use passengers' contact details for "sales & marketing purposes" - for example, advertising extra products. Despite these rules, you might find that in rare occasions, airlines still send these kinds of communications