I think the accounts (i.e. settings, passwords etc.) are purely synced, and not the emails themselves.
The actual syncing of the inbox probably only happens if you connect to the mailserver, and start syncing with that.
.Mac doesn't sync accounts, however it will sync your .Mac folders and smart folders with your other macs. I'm not sure it works with other mail services (POP, etc). You must enable it in the .Mac system preference panel.