From what I see.. you want to be able to access that computer form the outside
Go into your router, and make a DMZ, it's under the advanced tab (or something like that)
Enter your computer's internal IP there.
What this does is.. it routes all external connections to that comp, instead of the router, or of the comp it is configured for now. That'll let you receive email from the outside. Though be warned.. people will be able to portscan you.. and your computer will not be procteded by the router. Much less secure. I'd run some kind of firewall on that computer. (on all, but especially that one) Check out tldp.org for a tutorial on securing linux.