Firstly, I presume they can access other hosts on the Corporate network using the Cisco client ?
There is no configuration of the Cisco client by the user, other than the address and password (pre-shared-key) to connect to the VPN device, which can be a router, firewall or concentrator. It is on the VPN device that you need to look, and make sure that the subnet of the mailserver is included in the access-list that is used to define the traffic allowed over the VPN connection. You also need to check that the subnet that is used to allocate addresses to the VPN clients, has a route to and from the subnet of the mailserver.
I do not know what the Sprint broadband cards are, is this a GPRS card, to access via the cell phone network ? If it is, and the answer to my first question was ‘NO’, then make sure that NAT-Traversal is configured if you are using a firewall/ASA or concentrator. On the firewall/ASA just add the command ‘isakmp nat-traversal 20’ which may fix the problem. On the concentrator I think it is in the group config, and is a check box.
Try these things, and if it is still not working, give some meore information about the VPN device you are using, the configuration, and the IP addreses used, then we can probably help with a more specific answer.