However, most bounce messages have historically been designed to be read by human users, not automatically handled by software. They all convey the same basic idea ("the message from X to Y could not be delivered because of reason Z") but with so many variations that it would be nearly impossible to write a program to reliably interpret the meaning of every bounce message. RFC 1894 (obsoleted by RFC 3464) defines a standard format to fix this problem, but support for the standard is far from universal. However, there are several common formats (e.g., RFC 3464, qmail's qsbmf, and Microsoft's DSN format for Exchange) that cover large proportion of bounces.