Multi-user email management system

A group of users should read and answer some common email boxes. Such activity becomes harder when users move from office and need to know if someone else has already read and answered the message they are reading. So, we are asking GAF community for a web application that: 1) catches all messages from some given email boxes; 2) organizes them all together on an easy to use interface; 3) when the first user reads a message the message will be classified as "readed by [userName]" from then on; 4) when a user answers an email, that email will appear as "answered by [userName]"; 5) when a user reads an email, he may also choose "to assign" it to another one, giving it a priority by stars (1 to 5): so the message from then on will appear as "assigned to [userName]" with "[1 to 5] priority". These should be the main rules of the system. Of course it will be welcome any further improvement. I mean, for example, some filters that enable users to see only: "high priority messages" "unreaded messages" "unreaded messages from a specified email box" "assigned messages to him or to another user" "yesterday unreaded messages" and so on... I am available to furnish further specifications and some layout, if needed. About backoffice area it should be very simple: 1) add/modify/delete users; 2) add/modify/delete pop3 email boxes. Thanks for your attention.