Can an email go 500 miles in 2025?

> There’s a lot to the story that’s obviously made up...

Obviously? I think I've had this phone call myself a few times, although in my experience it was never from a statistician and they didn't give me as much data, but I'm pretty sure the story is mostly accurate.

> I think this is nonsense... why would an invalid or incomplete sendmail configuration default to three milliseconds?

This is a wonderful question, and perhaps much more interesting than anything else in the page, but first, let's reproduce the timing;

My desktop, a 2017 Xeon E7-8880 (144 cores of 2.3ghz; 1tb ram) with a load of 2.26 at this moment:

    $ time sleep 0.001
    real    0m0.004s
    user    0m0.001s
    sys     0m0.003s

On my i9-10900k (3.7ghz) current load of 3,31:

    $ time sleep 0.001

    real    0m0,002s
    user    0m0,000s
    sys     0m0,001s

(In case you think I'm measuring exec; time /bin/echo returns 0's on both machine)

Now as to why this is? Well in order to understand that, you need to understand how connect() actually works, and how to create a timeout for connect(). Those skilled in the art know you've got a number of choices on how to do it, but they all involve multiple steps because connect() does not take a timeout as an argument. Here's one way (not too different than what sendmail does/did):

    fcntl(f,F_SETFL,O_NONBLOCK);
    if(-1==connect(f,...)&&errno==EWOULDBLOCK){
      fd_set a;FD_ZERO(&a);FD_SET(f,&a);
      if(!select(f+1,&a,&a,NULL,{.tv_sec=0,.tv_usec=0})) {
        close(f);return error;
      }
    }

If you read this carefully, you only need to ask yourself how much time can pass between the top of connect() and the bottom of select(), and if you think it is zero like tedu does, you might probably have the same surprise: Computers are not abstract machines, but made out of matter and powered by energy and thus subject to the laws of physics, and so everything takes time.

For others, the surprise might be that it's still 3msec over twenty years later, and I think that is a much more interesting subject to explore than whether the speed of light exists.

Never got this, honestly.

Well, first light does 500 miles in 3ms, but the connect signal needs to come back, right? So it should be 250 miles, at most? But this is just a detail.

More importantly, because it seems to assume that all other operations besides the signal actually reaching the destination are instantaneous. As you point out yourself, computers are not abstract machines, so the actual response time between the signal being received by the destination (even assuming it's just one straight line with zero electronics in between) and the destination replying is not zero. I imagine there can be a large variation between physical installations and different types of hardware, so much as to make it very hard to detect a clear 500 miles boundary.

Or am I missing something?

The author wrote an FAQ several years after the original story that answers most of your questions. https://www.ibiblio.org/harris/500milemail-faq.html

Yes I think I had read those FAQ at some point, they're terribly handwavy though.

"Should have been 6ms instead of 3 for the ACK to come back? Yes, sorry, it was too boring to add"; "Should it be much more and variable because of the routers in between? Yes sure, I probably pinged them and added up the delays"; "Shouldn't plenty of deliveries have failed for destinations much closer than 500 miles? Yes sure, but that must have been the limit..." Etc.