We have a simple Windows Server 2003 Domain with 5 users. We are behind a SonicWall firewall and a WorkGroup Switch. We use a third-party ISP for basic POP3 email within Outlook Express.
The issue is that we cannot send emails to other users within the network but we can send out and receive from the outside. The sender receives no message back and the receiver receives nothing. Users cannot even send/receive emails to themselves.