JetBlue doesn't have a specific "Rotterdam office," but serves the Netherlands via Amsterdam Airport Schiphol (AMS), with a local number for baggage inquiries (+1-866-829-1086), a toll-free number for the Netherlands (1-866-829-1086), and their main website/contact options for other needs. You'll find their check-in at Terminal 3, opening 3 hours before flights, and can reach them online for support.
