In most cases, Exchange Servers are placed on the internal LAN with the rest of their domain. This is the recommended configuration behind a SonicWall. Exchange requires extensive ties between other nodes in the domain, such as the domain controllers, WINS servers, and clients. Getting all of these connections to work properly over any firewall and/or NAT device can represent a serious challenge.
If you are experiencing connectivity issues, check your WINS/DNS settings. Name resolution is usually one of the main problems encountered. The WINS/DNS settings will affect the connection to Exchange. If you use the IP address of the exchange server, the exchange server reconfigures the client to use the name. Once this happens, the client may have a problem with resolving that name. See https://mskb.pkisolutions.com/kb/837391 for additional details. See Exchange 2000 Windows 2000 connectivity through firewalls for firewall related Exchange issues.