And today's web mail no longer uses 80. https is another port. You can find that at http://en.wikipedia.org/wiki/HTTPS
I'll skip to why that port doesn't need to be opened by routers. Today's routers use a dynamic NAT system. I won't write at length here since all that is on the web but there is no reason to tinker much here.
-> In short, just a NAT router (almost all are that!) drops incoming unsolicited packets. That's why we didn't need another firewall. Some of your newer networkers add more firewalls then explode when it fails.
If I understand correctly, services listen to ports. Meaning a mail server may listen on port 80.
However, what about the application? If I am running a mail client, what port does it listen on after it sends a request out to port 80 on the mail server?
Why doesn't that port need to be opened in default router firewalls?
Or, why isn't it a security risk to leave that port open?