UTF-7

UTF-7 (7-bit Unicode Transformation Format) is a variable-length character encoding that was proposed for representing Unicode text using a stream of ASCII characters. It was originally intended to provide a means of encoding Unicode text for use in Internet E-mail messages that was more efficient than the combination of UTF-8 with quoted-printable.

Motivation

MIME, the modern standard of E-mail format, forbids encoding of headers using byte values above the ASCII range. Although MIME allows encoding the message body in various character sets (broader than ASCII), the underlying transmission infrastructure (SMTP, the main E-mail transfer standard) is still not guaranteed to be 8-bit clean. Therefore, a non-trivial content transfer encoding has to be applied in case of doubt. Unfortunately base64 has a disadvantage of making even US-ASCII characters unreadable in non-MIME clients. On the other hand, UTF-8 combined with quoted-printable produces a very size-inefficient format requiring 6–9 bytes for non-ASCII characters from the BMP and 12 bytes for characters outside the BMP.