Removing information from a hard drive or server may seem like a simple one-click procedure, but permanently deleting data is all but impossible without military grade software.
With the correct software, forensic investigators can sift through broken data and rebuild it.
When you send someone a message it will travel through - and be stored on - a whole host of servers, from a sender's hard drive, to a company's server, through various email gateways and then on to the recipient's server and hard drive.
The only way to delete such an email permanently would be to wipe it at each and every one of those steps.
If investigators have access to those steps they can start to piece together the gaps.
