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Commercial and government computer networks are archived on a regular basis, typically daily. Multiple copies of the archive are stored in physically different places to limit the damage from natural disasters, fires, etc. The archives are maintained in different increments (daily, monthly, annual) based on a backup strategy, but something like a monthly snapshot of the system is retained for years, if not permanently.
As messages move through the system, copies get made. Consider companies that monitor employee email for content. That isn't done on a unique message that appears, moves through the system, then vanishes. Copies end up in a variety of caches. It is the cache copy that is examined. Each of those caches is archived regularly.
Finally, the word "delete" is hard to define. In your home email account, delete may well mean that a message is erased. It might still be recovered through heroic efforts, but basically, it's gone. In a commercial or government setting, that kind of delete could be a problem. What if you delete a message from Tony Blair before you get a chance to respond? Touchy. Delete in commercial systems generally means "mark as read" and so it becomes invisible to anyone but a sysadmin who can make it magically reappear.
Deleted, as Karl defines it, if purest fantasy.
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