Charlie,
I don't think it is hardware, I've seen the same problem on two separate networks, both using cable. The internal network works fine, and as previously stated, the external interface works for delivering mail. It seems to be a routing problem. The nearest thing that I can figure out is that it happens when DHCP goes to renew the lease on the external interface. A restart of the server always solves the problems. On the machine in my house, I even did a clean install of 5.5 to replace 4.12 on the server, but the problem persists. It has quieted down in the last few weeks (it has happened only once), but before that was happening daily for 3 weeks. I suspect it is on the other end, rather than an E-Smith problem.
Bob