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Sendmail's case folding and errno reporting produced the “Not a typewriter” error

unsung.aresluna.org Editor score 6/10

Sendmail lowercased a mixed-case username, then appended a system error value to “User unknown,” producing the puzzling message. It is a useful example of how layered error handling can obscure a system's actual reasoning.

What

- A sendmail error for an unknown email user could end with the confusing phrase “Not a typewriter.”

How

- Sendmail lowercased usernames, then appended the system error string, which described a background process with no user I/O.

Why it matters

- The case shows how combining an application failure with an unrelated system error can mislead users.

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