A hosted exchange solution? Where all of you can have your own mailbox as well as full rights to a "service box" where you all have read/send permissions? You can all use MS Outlook and it will be visible in there and you can use checkboxes and color the emails after they have been replied to, and it will be reflected for all of you.
See more here:
http://www.myhostedsolution.com
Only $9.95 per month per mailbox, and that is for unlimited traffic and mailbox storage. Been with them for a long time, they're great. Many will offer hosted exchange, but few will give you large mailstorage, many will put a limit at 2, 5 or 10 gb per mailbox. You even get one MS Outlook license per mailbox, so that you all can use the full features of Exchange.
Hope it helps!