Dropbox employs a somewhat unusual technique to lower its costs, the cloud software company revealed on Friday in its filing to go public . From a report: In a process the company calls "infrastructure optimization," Dropbox said it deletes users' accounts if they don't sign in for a year and don't respond to emails. That keeps the company from incurring storage costs for inactive users, a tactic Yahoo has used in the past. Dropbox said that the costs of revenue dropped 6 percent in 2017 to $21.7 million, mostly due to a $35.1 million reduction "in our infrastructure costs." As it prepares to lure public market investors, Dropbox is paying particularly close attention to its expenses. The company operates in an intensively competitive market against vendors including Apple, Amazon, Box, Google and Microsoft. Once reliant on Amazon Web Services , Dropbox has moved away from public cloud in recent years and has been building its own data center infrastructure to store the majority of user data. Another way it's managed costs is by making sure that there weren't too many copies of users' files on third-party infrastructure.