hi
AFAIK for accessing a mail server which is in the outside network u only need to have global,nat statements with route which gives u the default route statement to the outside interface.
but if you gotta mail server in the inside lan and want to give the access from the outside public network then u need to allow acess on the particular port in which the services are running as well as the mapping to an external ip address.
in this case u need to use up static as well access-list,acess-group commands to achieve the same..
i donno about which mapping you are talking over here for accessing the outside mail server..
would suggest to check these links for more info on the same..
Outside Mail Sever Access
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_configuration_example09186a0080094467.shtml
Inside Mail Server Access
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_configuration_example09186a0080094466.shtml
regds