They were Windows guys through-and-through. Our ISP had provided a Solaris box that sat in our server room between our network and the world. Or it sat there until they decided that since they didn't know what it was doing it must not be needed so when upgrading some switches they left the yellow box out of the loop.

Shortly thereafter our clients started reporting that they weren't getting email that our attorneys said they had sent. A little investigation showed their systems were bouncing it because our domain had been blacklisted for spam. We were an open relay.