I use Imanage Mailsite(now called Interwoven) to save important emails. The emails are all full-text indexed and easily searchable. Its expensive though if that is all you would need it for. There are cheaper document management systems, like Worlddox, but I'm not sure they all work with email.
I also archive emails through Outlook. Generally I would do this if someone left the company and had lots of miscellaneous email. The search capability in Outlook really isn't that robust, but it would work. I'm not sure what email system you are using, but they all have some kind of archive capability.
Its important to have some kind of email retention policy for compliance reasons because of the Sarbanes-Oxley act, SOX for short. Some companies opt to not save emails after a certain date because you can be forced to produce them in the event of some kind of legal action.