A statistics department at the University of North Carolina at Chapel Hill could reportedly send email no farther than roughly 500 miles. A vendor OS patch had downgraded Sendmail 8 to Sendmail 5 while leaving the Sendmail 8 configuration in place; the older binary ignored the long-form option names it did not understand and had no compiled-in defaults, so the SMTP connect timeout became zero and aborted after about three milliseconds. On a low-latency fibre network, round-trip time tracked distance closely enough to produce a delivery radius. The account is first-hand and its author stands by the behaviour, but he states in his own FAQ that the narrative was altered for effect, that the three-millisecond figure was not the connect timeout, and that no logs, notes or configuration survive.
Editorial judgements on a 0 to 10 scale, argued in the notes below and nowhere else. They order a list. They are not evidence and they never appear in a claim.
Put the round-trip light-speed floor in every latency budget before you tune anything else: about 1 ms per 100 km in fibre. A budget under that floor is not a timeout, it is a distance limit.
Separating the parts, which is the whole value of this record. OBSERVATION, supported. Trey Harris, named and reachable, ran the centralised campus email system Isis at UNC Chapel Hill and wrote the sendmail.cf files used on most campus mail servers. Asked directly whether it happened, he answers 'Yes, it happened.' There is no second witness and no artefact, so this rests on one first-hand account by an identified practitioner. EXPLANATION, hypothesis. The mechanism as told is plausible and is the author's own reconstruction, not a measured finding. He states he fixed the problem by replacing the binary as soon as he knew that would work, and kept the old one to experiment with afterwards. WHAT THE AUTHOR HIMSELF WILL NOT CLAIM, all from the FAQ. The date: 'I wish I could say for certain', placed only 'somewhere between 1994 and 1997' from office and coworker memory. The evidence: 'I didn't keep logs of what I found, and I no longer have the notes I took at the time.' The arithmetic: asked why three milliseconds cannot be a connect timeout, he answers 'Yes, I know. And it wasn't the timeout, actually', and concedes the round trip would make it at least six. The dramatic reveal: he already knew the speed of light was involved before running units, and worked it out backwards on purpose. The switched-network detail: 'Feel free to delete that paragraph mentally upon second reading.' The vividness: 'I took license. It made a better story that way.' UNVERIFIED SUPPORTING CLAIMS. That Sun's Sendmail 5 would accept a Sendmail 8 configuration at all is asserted from the outcome, not from source: 'If you have access to that source, I'd love independent verification.' That sendmail assigned zero rather than a compiled-in default rests on a remark he recalls Eric Allman making in a LISA tutorial, which is hearsay about the general case rather than evidence about that binary. REPRODUCTION. Not possible as told: no date, no exact versions, no configuration, no logs. The mechanism class, a zero connect timeout turning propagation delay into a delivery radius, is reproducible synthetically and that is a separate experiment worth writing. It would support the mechanism under stated conditions; it would not corroborate this incident. WHY THIS RECORD EXISTS. The story is usually forwarded as a case study. Its own author declines that reading twice, in the original post and again in the FAQ. The interesting artefact here is not the 500 miles, it is a well-documented instance of an engineering anecdote hardening into fact while the person who wrote it keeps saying it should not.