If the email is on a server that’s backed up, there will likely always be a copy of it, ‘somewhere’.
If the email has been downloaded onto your computer and saved in a ‘Personal File/Folder’, it would take a catastrophic failure of the hard drive to eliminate it, and I mean CATASTROPHIC. We’re talking, physically destroyed. I suppose it’s also possible to do a thorough ‘wipe’ of the drive.
Even in the latter case, if the message was on the server before being downloaded to the computer, chances are it was backed up and is being stored within the backup.
Funny you should mention catastrophic failure!
After the hard drive predictive failure analysis.
The drive was deemed to fail and shredded. The shreds were mixed with many others and sent to a secure recycling facility.
A virus had also damaged all the other drives where the emails had been!