Pavel Curtis is a familiar name to me.. he wrote LambdaMOO! But here, he's just annoyed at email addresses:

I attempted to reply to a news item recently for which the return address was as follows: vax135!harpo!mhtsa!ihnss!ucbvax!C70:sri-unix!KING@KESTREL I have now received my letter back from vax135!harpo!mhtsa!ihnss!ucbvax!Network:c70 telling me that there was "No such mailbox at this site"

Could someone please tell me just what the address means and how it gets generated and how I'm supposed to reply to it?

The explanation was complicated; the address tried to traverse both Usenet and the ARPANET, and failed. Even parsing it was ambiguous.

Following up to that was this awesome post:

So you want a reply command that works 100% of the time? So does the rest of the world! The UUCP/Berknet/ARPA environment is just too weird and full of glitches for any static program to handle it. Berkeley's Mail program makes a valiant attempt but botches about half the time I try to use the reply command.

End-to-end communication that works all the time. Yes, that would be nice. :)

While I've read repeatedly how Sendmail's baroque syntax was designed to allow for translation of email addresses between networks, I didn't really understand the motivation until I saw this thread play out on Was this thread perhaps an inspiration for sendmail? It will hit the sometime this year.