Thanks for your help. I got it fixed. The firewall solution we are using (IPCop) somehow reverted to its original /etc/hosts files. Here's our setup:
ISP > IPCop > SME & local network
The IPCop has an external IP of 204.x.x.249 and an internal of 10.0.0.1. The SME is 10.0.0.249. Our DNS record with our ISP says that the IP of our domain should be 204.x.x.249, which is fine for the rest of the world. Ports 80, 443, 25, and 110 are forwarded through the firewall to SME. For internal machines, the IPCop provides a few local DNS entries through its hosts file--www. mail. and ftp.myschool.k12.mo.us. So, normally the local email clients would connect to 10.0.0.249 to send mail. Since the hosts file on IPCop was reverted somehow, the clients were connecting to 204.x.x.249 to send mail. Therefore, SME thought the clients were connecting from a network outside our own.
Thanks for your help.
Ryan