|My suggestion is to use the IMAP protocol to connect to your email. Windows Mail and Outlook Express are both IMAP compatible, you just have to make sure whoever hosts your email account that they support IMAP.|
In short, IMAP is different from POP in that emails remain on the server and it syncs the main folders, and I believe it also syncs the subfolders, too. So what you can do if you have 2 people accessing the same mailbox is create 2 subfolders under the Inbox, example \Agent1 and \Agent2, and have the agents move the messages they are going to be working on into their own respective subfolder so they don't "step on each others toes", per se, or don't work on messages at the same time.
Read this Wiki article on IMAP to get a better idea.
Another idea may be to have the agents access the mailbox using a web browser instead of an email client. They may have to refresh frequently to see changes, but the changes would be more immediate than IMAP, which may take a little while to sync changes. I setup a small online pharmacy company this way and it worked best for them.
Assume that I already did an Internet search.