so i got a situation that user shouldn't purge his deleted mail, and i never knew about this before.

can we really do that? and how?

Thanks and Regards.

You can't prevent user to purge the items from deleted items but you can enable the Litigation Hold so it will keep the items in Dumpster 2.0 to recover and search during discovery...

Litigation Hold and In-Place Hold in Exchange 2013 and Exchange Online - http://blogs.technet.com/b/exchange/archive/2013/12/11/litigation-hold-and-in-place-hold-in-exchange-2013-and-exchange-online.aspx

In-Place Hold and Litigation Hold - https://technet.microsoft.com/en-us/library/ff637980%28v=exchg.150%29.aspx