ProtonMail is as secure as email gets. But due to how web systems work and the nature of email, ProtonMail can monitor the IP address from which you access its service. This could potentially be used to identify you. However, since it’s a Switzerland-based service, you’re well protected by iron-clad privacy laws. And if you use a VPN, ProtonMail can only track the IP address assigned to you by the VPN provider, not your actual IP address.