Especially if the data is backed up on multiple striped RAID data storage devices.
There is also the much more time consuming, tedious, and man-power-intensive method of searching through the email servers of suspected email recipients or email senders.
Unless emails are internal, there is a copy of each email on two different email systems. The sender’s and the recipient’s.
At least that is my understanding.
Plus, unless the emails were just to others on the same server, those who received or sent the emails and servers in between -and foreign intelligence agencies- could have multiple backups.