"Traditional, but Puzzling"

A common Unix-era mystery - a bounced email showing "550 mwichary... User unknown: Not a typewriter" - is explained as the product of four interacting quirks. Sendmail quietly lowercased a capitalized username, producing a non-existent account and the correct "User unknown" reply; then the program appended whatever the system error code supplied for extra context. That system error, ENOTTY, literally read "Not a typewriter" because early computers used typewriters and teletypes (tty) for I/O. The terse historical string survived long after teletypes vanished, and the colon joining the messages made it read as if the recipient were being labeled a typewriter.

The episode is used to show how historical hardware, terse debugging habits, and small implementation choices leak into modern user-facing messages. The error message has since been reworded to "Inappropriate ioctl for device," but the error code ENOTTY remains, and similar legacy terms still permeate systems today. The case illustrates why seemingly baffling or oddly phrased errors often have straightforward roots in backward-compatible code, compact error strings, and the long memory of system interfaces.