Since its introduction, Workflow for Microsoft Dynamics GP has proven to be very popular and is a module I have spent a lot of time with. Workflow integrates with the Web Services for Microsoft Dynamics GP for the approval (or rejection) via email; there is a fair bit of complexity around this area, mainly in accessing the email actions from outside of the domain.
Dan Peltier of Microsoft has a good write up of what is required for DNS settings to allow for external access of approvals.
In the near future, I am going to be doing a small series on configuring web services to use https for approval/rejection, but will be written on the assumption you have working DNS records ready (DNS is very much not my specialist subject). Dan's post is a good starting point if you need assistance in this area.