Cops recover deleted data through use of specialized forensic software, which scans computer and devices for remnants of any information that has not yet been overwritten.
The police look for data in the device's storage, which the operating system marks as free space after the file or files are deleted, but not erase immediately.
The process is called digital forensics and can also involve checking cloud backups as well as using file carving and other specialized techniques.
The only true way of destroying the data is to remove the hard drive and destroy the hard drive to make it unrecoverable or if in a tablet or phone you would need to fully destroy the tablet, phone or other device.
Cops and police can also track your email if they have a warrant or subpoena to do so.
With the warrant or subpoena the cops and police can track your email by analyzing of the full email headers to find out the email sender's IP address and also examine metadata and also request any email records from internet service providers.
The police can also access any stored emails on cloud storage and devices and the police may even cross reference the data with other databases to build a comprehensive case.
Although without the warrant or subpoena, the police cannot just simply access your email or track your email.
The full email headers also contain any technical information, which includes the sender's IP address, which can also be crucial for tracing the origin of the emails.
And with legal authorization such as a warrant or subpoena, the police can ask the internet service provider to link an IP address to a specific customer account and location.
Police also can use forensic tools to access and recover emails that might have been deleted from cloud storage or a user's device.
And tracing the email is also easier when the sender of the email also includes personal information in the body of the email or when the sender of the email uses a personal email address, as it can also provide direct identifying information.
And police and law enforcement can also combine the email data with other information, like criminal records or location data to create a more complete picture of the sender of the email's connections and activities.
Using a VPN or Tor can make it much harder and much more difficult for police to trace a sender's true IP address and the email sender's information like the "From" field can also be faked.
But even then police can often still track you, but it can be much harder.