On Premise Exchange provides a personal archive. Do I need a separate email archiving solution?

Yes. A separate email archiving solution will ensure that all of your emails are being archived. On Premise Exchange provides only personal archiving capabilities, meaning any user can delete the emails from a personal archive just as they delete the emails from their primary mailbox. So, if you are trying to preserve the emails then you will have to place a legal hold on all the mailboxes.