Do virtual servers explain the loss? That is, an email server farm sitting behind a load-balancer that appears as a single IP address but only some servers are configured to reply to bounces?
We try to check for this phenomenon by examining and recording the greeting banners. Indeed, we see virtualized servers, but loss from these domains is not correlated to a single server (identified by a unique hello banner). • Q: Couldn’t a server be configured to respond to bounces differently? Or perhaps differently when under load? A: This certainly could be true. However, we do not know of a real-world system or architecture that would explain this. Recall that the confusing results are the losses, emails for which we never get a reply. To explain our results, the system would have to be set up such that under load, the server probabilistically drops some fraction of bounce replies (or doesn’t generate the bounce message). If someone feels that a large number of MXes are configured this way, we’d love to hear from you. • Q: Could spam filters explain the losses? A: Again, this could be true but we are yet to find a real architecture or system that would do this. For example, the sp
