Legal reasons for having mail archiving and retrieval (UK)

I work for a UK-based company that exports around the world. At the moment we’ve got a UNIX-based mail system which is pretty basic; just POP3 and SMTP with mail handled by Outlook and stored locally in PST files. It’s OK but has limitations.

I want to move the system over to Exchange 2003 later this year and would also like to have some sort of email archiving and retrieval system in place. However, I’m trying to find sources of unbiased legal info on what the company HAS to have in place so I can create a report to the Board with a proposal on the new email system.

Does anyone on here know of some good sites where I can get UNBIASED info?