Dropbox has enlisted external security professionals to examine why users have begun receiving spam.
More than 100 posts emerged from users who reported that spam was hitting their dormant Dropbox email accounts.
Users insisted their accounts had not been used for other registrations, to send email or linked to other applications. Others denied the possibility that their machines that they used to access Dropbox had been compromised, while others speculated that the company had been hacked.
The spam appeared to be sent from European domains and was mainly Casino scams.
Dropbox engineer Joe Gross said that the company's security team was investigating and had drafted in a team of outside experts.
Gross said: “While we haven't had any reports of unauthorised activity on Dropbox accounts, we've taken a number of precautionary steps and continue to work around the clock to make sure your information is safe."
However he denied concerns that a brief overnight outage was indicative of a data breach, noting it was "incidental and not caused by any external factor or third party".
Eric Chiu, president and founder of HyTrust, said: “Dropbox is a great example of an application that has infiltrated the enterprise, which can have serious security consequences since employees are hosting corporate confidential data without any enterprise security controls.
“With external and internal breaches happening daily, this is a perfect formula for major disasters to happen. The need for consistent configuration, and controls for access, management and visibility are critical.”