Hi Martin,
I think you'll be better off using static nat on the 827 router, I am not sure if the Web Interface allows you to configure static nat, so what you can do is, telnet to this device and configure static NAT. Follow these steps,
Telnet to the router, go to enable mode and then go to the config mode and type the following command
ip nat inside source static i.i.i.i o.o.o.o
where i -> is your inside mail servers ip address
o -> is the IP address that the DNS resolves for mail.yourdomain.com.
you can repeat the above command for your web sever also (if you have one) with i as internal web severs ip and o as outside ip that DNS resolves it to ...
Hope it helps and let me know if you need more details
Aslam.