WebAccess has two components: WebAccess Agent and WebAccess Application. If you're web server is on a different box from the GroupWise domain/MTA and WebAccess Agent, the WA agent talks to the WA application/web server on port 7205 inbound and dynamic ports outbound.
If the WA agent and the WA application/web server are on the same box, they will happily talk together...again using port 7205.
Users will need both port 80 and 443.
If the WA agent is on a box different from each of your post offices, it will need port 1677 access to each inbound and outbound.
Hope that helps.