Spotify, the online music streaming service, notifies via email about suspicious activity of users who use unauthorized or modified versions of the Spotify app for free access to Premium features.

These hacked apps allow freeloaders to skip songs indefinitely and enjoy streaming without ads – features that are available only for premium users.
The free version of Spotify has certain limitations, such as ads, shuffle playback, bypassing restrictions, and various such options that encourage users to purchase the premium version. These modified versions of Spotify make the premium version unnecessary by allowing users to enjoy unlimited streaming with the help of installation files that can be downloaded together with the application.
Spotify sends an email to users whose accounts detect any “abnormal activity” and warns that future violations could lead to suspension or even termination of the Spotify account.
According to the email, the user must disable the Spotify app and re-download the official app from the Google Play Store.
It hasn't been revealed how many users are using these versions to enjoy free, unrestricted streaming. According to figures released by the company in December, the service is used by more than 159 million users worldwide – 88 million of whom are users of the free Spotify tier.
Taking into account the company's current losses, it is not surprising that they are finally addressing the issue.

