Since chatter sends email, why not send the emails via the contact chatter? When you select the full pop-out button in the "send message" box, you even get the choice to select an email template. So, you could design a template that looks like any other email from a sales person and have the sales team use that every time they contact a client, and everything will be tracked in the chatter.

Dear Shawn Wright,
We know we can track messages and emails send from odoo itself but what we want is that we wanted to track emails of the saleperson that he sends from his inbox suppose from outlook inbox to the client inside the clients lead!