Veiled in clouds? Assessing the prevalence of cloud computing in the email landscape

The ongoing adoption of cloud-based email services — mainly run by few operators — transforms the largely decentralized email infrastructure into a more centralized one. Yet, little empirical knowledge on this transition and its implications exists. To address this gap, we assess the prevalence and exposure of Internet users to cloud-based email in a measurement study. In a first step, we study the email infrastructure and detect SMTP servers running in the cloud by analyzing all 154 M .com/.net/.org domains for cloud usage. Informed by this infrastructure assessment, we then study the prevalence of cloud-based SMTP services among actual email exchanges. Here, we analyze 31M exchanged emails, ranging from public email archives to the personal emails of 20 users. Our results show that as of today, 13% to 25% of received emails utilize cloud services and 30% to 70% of this cloud usage is invisible for users.