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Quoted-Printable, or QP encoding, is a binary-to-text encoding system using printable ASCII characters (alphanumeric and the equals sign =) to transmit 8-bit data over a 7-bit data path or, generally, over a medium which is not 8-bit clean. Historically, because of the wide range of systems and protocols that could be used to transfer messages, e-mail was often assumed to be non-8-bit-clean – however, modern SMTP servers are in most cases 8-bit clean and support 8BITMIME extension. It can also be used with data that contains non-permitted octets or line lengths exceeding SMTP limits. It is defined as a MIME content transfer encoding for use in e-mail.
QP works by using the equals sign = as an escape character. It also limits line length to 76, as some software has limits on line length.