[pontifier]

This is absolutely one of the key formative stories that helped me to think about systems at light speed scales.

I'm currently at the very early stages of building a science museum and will eventually try to incorporate this story into an exhibit about light speed. This along with "Nanoseconds", foot long pieces of wire like what Grace Hopper handed out, can truly help to bring this topic to life.

I'm also attempting to use this as the basis for a blockchain based "proof of proximity" in which a very high number of round trip encryptions of the previous blocks hash are stored in a bitcoin block. The number of round trips would be high enough that devices even a few hundred feet apart couldn't complete the task before the next block.

[TurkishPoptart]

I actually don't understand the last part of the OP story - can someone explain it?

[pontifier]

The sending mail server was configured with a zero timeout for connections. If it didn't IMMEDIATELY get a response from the destination mail server it would fail. This immediate failure took 3 milliseconds, a long enough time that some servers could actually respond back before that happened... but if the server was too far away (more than 500 miles) the connection would fail before the first packet could even get there due to the finite speed of light.

[chriselles]

As a tech/coding newb I was assuming it related to speed of signal travel to approximate 500 ish miles.

Thanks so much for confirming it.

I‘m going to share this one with my sons who will appreciate the humour.

[gurjeet]

Thanks for writing the TL;DR version!