Hi, (we previously posted this on the e2e mail list; apologies if you are reading it for the second time) We're looking for operational-types lurking on the list with experience running large mail servers. In particular, we have collected a large amount of data as part of an Internet email performance study that we cannot entirely explain. If you can help us or are simply curious about our findings, we'd love to hear from you. WHAT WE DID: Briefly, we used SMTP bounce-backs as the basis of an email active measurement survey. Using random addresses as unique identifiers, we measure latency, loss, paths, etc. to a large set of Internet MTAs. Approximately 1/3 of all servers we surveyed respond with bounce-backs. We've found some interesting results. For example latencies of days (30 days in once instance). WHAT WE DON'T UNDERSTAND: Most servers behave as we expect, either always replying with bounce-backs or never replying. However, some exhibit odd and seemingly non-deterministic behavior. For example, a server will respond to all emails for weeks, and then reply to only a fraction (e.g., 25-75%) of the emails in a seemingly random pattern for some period of time (e.g, 4 hours). Further, we often see these patterns correlated within a domain (e.g., a subset of the MTAs will enter and exist this loss mode at the same time). We are fairly certain that the loss is an artifact of the MTA behavior or local administration. While we can guess reasons this might occur, we have yet to find an administrator who can explain this behavior with an architecture used in practice. More details on the project including our exact methodology, plausible explanations for the loss and a FAQ are available on our web site: http://ana.lcs.mit.edu/emailtester Thanks! Rob Beverly / Mike Afergan