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This is not a defense of the WH staff, but it is possible to delete an e-mail and not have a copy of it hanging around. If the WH uses a client/server e-mail system (such as Microsoft Outlook and Exchange Server), a user who connects to their mailbox on the e-mail server with their client does have control over their own mailbox on the e-mail database on the server. If they send or receive an e-mail, delete it permanently before the e-mail database on the server is backed up to a another location or medium, and do not move a copy of it to their local PC or network share then there is not a copy of the e-mail just sitting around on the server. Of course, everyone who has a copy of the e-mail would have to do the same thing for it to be completly gone.
Now, if the WH is using an automatic e-mail archiving system which would move a copy of every e-mail sent and received in the system to another location, the previously described scenario does not apply. The copy of the e-mail would also have to be deleted from the archiving location for it to truly be "gone".
Also, if the written policy (if strictly followed) of the WH to delete all e-mail after 30 days falls within the stated period of the laws that cover their electronic communications, they have not done anything legally wrong by setting 30 days as the time period for retention.