This growth has led police and intelligence agencies to warn that they are struggling to intercept communications by criminals and terrorists: privacy campaigners argue that states are already infringing too much on the privacy of individuals and that encryption is a vital way of preventing further erosion.
It is into this complicated mix that ProtonMail is coming out of beta, firmly on the side of the privacy advocates.
"Strong encryption and privacy are a social and economic necessity. Not only does this technology protect activists and dissidents, it is also key to securing the world's digital infrastructure," ProtonMail co-founder Andy Yen said.
"This is why all things considered, strong encryption is absolutely necessary for the greater good."
ProtonMail said in light of "recent challenges against encryption and privacy" it has decided to open the service for public registration, so anyone who wants an encrypted email account can obtain one immediately.
"The best way to ensure that encryption and privacy rights are not encroached upon is to get the tools into the hands of the public as soon as possible and widely distributing them," Yen said. "This way, we put the choice in the hands of the consumer, and not government regulators."
The company, founded by a group of scientists who met at CERN and MIT, is based in Switzerland. Its service launched in beta in May 2014.
The initial funding for the company crowdfunding campaign raised $550,000. Its client-side code is open source and reviewed by its community.
The growth of ProtonMail reflects another problem facing governments that want to limit the use of encrypted communications. While legislation such as the UK's Investigatory Powers Bill will allow them to require local communications companies to remove the encryption they have applied, they will struggle to demand the same of companies based in other countries.