I have been asked this question quite a few times by clients and this also comes as a very frequent objection. To this I do a very simple analogy to explain: The latest backup systems come with real time backup sequence for the emails in your mailbox, if there is a crash, the backup system helps you restore your mailbox and usually done in a matter of minutes if you have best of breed and costly systems, otherwise it can take an administrator a few days to restore a few gigabytes of data. But now if you have accidentally or unintentionally deleted emails in your mailbox, you might not be able to retrieve it from you backup system say after a week as it has also been deleted in your backup.
On the other hand, an archiving solution is more a long term plan where emails can be stored over several years and kept readily accessible even if the email has been deleted from the end user point of view. So this is a simple example to demonstrate the necessity and importance of having an email archiving solution.
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