“Traditional, but puzzling”

In the 1990s, users of some Unix-based operating systems would sometimes see a peculiar error. It happened occasionally when sending an email to a person you might have already sent email to before, and in addition to the error being confusing, the message itself was particularly bewildering. Let’s say you wanted to send an email to “MWichary.” Instead of a confirmation, you would see this: 550 mwichary... User unknown: Not a typewriter As many strange errors tend to be, this one was at an intersection of four different things. The cause of the error was something many of you will recognize – code being anal about lower- and uppercase. Behind the scenes, a piece of software called sendmail always assumed lowercase user names, which was true early on. “MWichary” was quietly changed to “mwichary” – a user that didn’t exist – and the program reported the literal outcome, instead of its chain of reasoning. The second part was a sort of a meta-error in the error handling routine. Sendmail…