Shared mail account

My boss has just taken on a new assistant, who will be filtering all his mail for him. I am wondering if there is something simple that I can do so that what the assistant does (ie filing of emails in to folders for him read as appropriate (ie For Info, Private, Urgent) or refer back to, and replies, etc..), can be seen by my boss as well?

Is this an MS exchange type function? Both people use laptops, and so need an offline function if possible. Am I needing to look at IMAP? It is an area that I do not know at all, so any helpful advice and links would be much appreciated.

Justyn