"Traditional, but Puzzling"

Automated excerpt

In the 1990s, users of some Unix-based operating systems would sometimes see a peculiar error. Let’s say you wanted to send an email to “MWichary. ” Instead of a confirmation, you would see this: 550 mwichary... Sendmail correctly reported back “User unknown,” but then reflexively added whatever the system error value reported, for extra debugging color.

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