|This will most likely not work for a few reasons. |
1) Most ISPs typically block port 25 to home users to specifically prevent home users from running servers as well as to prevent the flow of mass mailing worms. Most outgoing traffic using port 25 MUST connect to the ISP's own mail servers or the traffic will be blocked.
2) You do not need to configure any MX records on your router, you would only need to forward traffic on the mail related ports directly to your server.
3) If you still wish to proceed, would recommend signing up with a DDNS provider like dynDNS or no-ip.com. They have a program that you can download and run on the server that will keep the A record that is created with the DDNS provider updated with the correct external IP address of your connection. This will enable to you get to your server using a regular host name instead of having to remember the IP.
4) If you can access the MX records, and you have set up the port forwarding, and you have tested that your ISP is not blocking the mail related ports, then you can try to ADD an MX record. I would suggest moving the tertiary record to the 4th MX entry, moving the secondary to the tertiary spot, moving the current primary to the secondary spot and creating your own MX record as the primary. If you did step 3 and signed up with a DDNS provider, then simply enter the FQDN of the DDNS host as the MX record. (Example: if you are using no-ip and have no-ip.net configured as your chosen DDNS domain and serverxyz as your server's DDNS host name, then you would enter serverxyz.no-ip.net into the MX record host name area.)
In addition to the above, I have not tried hosting email on a Windows server without using Exchange, so I cannot provide assistance as to what else you may need to configure on the server side of things to allow for the proper communication. You may also need to have RDNS and SPF records configured to allow mail to be sent to some remote domains. Also, you will most likely be blacklisted on the Spamhaus ZEN and PBL lists due to having a Dynamic IP address and also not being whitelisted to be sending un-authenticated mail from within your ISP's IP blocks. Be sure that you test your server and that you are locked down and are not an open mail relay or you will be blacklisted in short order. You also want to make sure that you are using good passwords and have a good firewall in place as mail servers are always targets for malicious hackers.
In short, if I would be extremely surprised if you were able to get this to work with your current setup.
IT Desktop & Network Consultant - MOS Master Certified, MCP, MCSA, MCITP - Windows 7, CCNA Certificate Pending, A+, Network +