MIT Story, Email Only Goes 600 Miles or Less?

Does anyone have a link to this old HN/blog story, where the author was MIT email support and their professors'/clients found only email recipients within 600 miles could receive their emails. Turned out the emails were limited by some bounce-back timer, I don't recall the specifics, but that window times the speed of light = 600 miles.

I'd love to share the link today so I don't sound crazy for mentioning a crazy tech bug involving the speed of light to a colleague.