ProtonMail Theoretical Security

In the best-case scenario for ProtonMail privacy, all your email communication happens with other ProtonMail users. This way the unencrypted metadata associated with your email activity never leaves ProtonMail's servers in Switzerland. The only way for the U.S. Government get a picture of who you're emailing is to:

1. Compromise your machine in some way (possible though much less convenient than just ordering Google to hand over the info).
2. Get a court order from the Cantonal Court of Geneva or the Swiss Federal Supreme Court to compel ProtonMail to hand over the info about who you are emailing.