I know that OpenERP does want the software to replace your email software. The internal inbound and outbound mail features are designed to to enhance communication internally and with clients (i.e. sending quotations, invoices, etc), and streamline communication (i.e. turning a question message sent to an info@yourdomain.com into a lead automatically).

I don't think it it is a good tool to replace a dedicated email client or service. There are modules/apps in OpenERP to integrate with Outlook and Thunderbird. Settings --> Apps then search for "email"

I hope this helps!