Quoted-printable
A MIME transfer encoding that represents text with mostly ASCII characters, escaping non-ASCII bytes as =XX hex sequences. It keeps the majority of the text human-readable in the raw message source.
Quoted-printable (QP, defined in RFC 2045 section 6.7) is an encoding designed for text that is mostly ASCII but contains occasional non-ASCII characters such as accented letters or special symbols. Any byte whose value is outside the printable ASCII range is replaced by an equals sign followed by two uppercase hexadecimal digits — for example, the é character (0xE9 in ISO-8859-1) becomes =E9. Lines may not exceed 76 characters, so long lines are broken with a soft line break =\r\n.
The advantage of quoted-printable over Base64 for text content is readability: an English email with a few accented words encoded in QP is still largely readable as plain text in the raw source, whereas Base64-encoded text is completely opaque. For text with a high proportion of non-ASCII characters (such as Japanese or Arabic), Base64 is more efficient and is typically chosen instead.
A mail client must decode quoted-printable content before displaying it. Missing or incorrect decoding produces text with =XX sequences visible in the body, which is a common symptom of a poorly implemented or legacy email reader. Mbox Viewer decodes quoted-printable transparently as part of MIME processing.
Related terms
A binary-to-text encoding scheme that represents arbitrary binary data using only 64 printable ASCII characters, widely used in email to safely transmit attachments and binary content.
The character encoding that specifies how bytes in a text part are mapped to readable characters. Common charsets include UTF-8, ISO-8859-1, and Shift_JIS; a mismatch causes garbled text known as mojibake.