What does ProtonMail keep?

ProtonMail keeps your metadata: "sender and recipient email addresses, the IP address incoming messages originated from, attachment name, message subject, and message sent and received times."

"We [ProtonMail] also have access to the following records of Account activity: number of messages sent, amount of storage space used, total number of messages, last login time."

Based on the metadata, you're probably in trouble if the authorities come snooping. All the authorities have to do is start checking out the accounts of some of the senders. They can probably find out who a sender is and pay them a visit.