Editing UTF-7 (section)

==Motivation==
[[MIME]], the modern standard for e-mail formats, forbids encoding of [[header (computing)|headers]] using byte values above the ASCII range. Although MIME allows encoding the message body in various [[character set]]s (broader than ASCII), the underlying transmission infrastructure ([[Simple Mail Transfer Protocol|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 [[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&ndash;9 bytes for non-ASCII characters from the [[Basic Multilingual Plane|BMP]] and 12 bytes for characters outside the BMP.

Provided certain rules are followed during encoding, UTF-7 can be sent in e-mail without using an underlying MIME [[MIME#Content-Transfer-Encoding|transfer encoding]], but still must be explicitly identified as the text character set. In addition, if used within e-mail headers such as "Subject:", UTF-7 must be contained in MIME [[MIME#Encoded-Word|encoded word]]s identifying the character set. Since encoded words force use of either [[quoted-printable]] or [[Base64]], UTF-7 was designed to avoid using the = sign as an escape character to avoid double escaping when it is combined with quoted-printable (or its variant, the RFC 2047/1522 "Q"-encoding of headers).

UTF-7 is generally not used as a native representation within applications as it is very awkward to process.  Despite its size advantage over the combination of UTF-8 with either quoted-printable or Base64, the now defunct [[Internet Mail Consortium]] recommended against its use.<ref>{{Cite web|url=https://www.imc.org/imcr-010.html|title=Using International Characters in Internet Mail |work=Internet Mail Consortium |date=1 August 1998 |archive-url=https://web.archive.org/web/20150907234243/https://www.imc.org/imcr-010.html |archive-date=2015-09-07}}</ref>

[[8BITMIME]] has also been introduced, which reduces the need to encode message bodies in a 7-bit format.

A modified form of UTF-7 (sometimes dubbed 'mUTF-7'<ref>{{cite web
| url          = https://doc.dovecot.org/configuration_manual/mail_location/
| title        = Configuration Manual
| at           = Sec. "Mail Location Settings"
| author       = <!--Not stated--> 
| date         = 8 February 2023
| website      = Dovecot Documentation
| access-date  = 2023-02-28
| quote        = Store mailbox names on disk using UTF-8 instead of modified UTF-7 (mUTF-7).
}}</ref>) was used in the [[Internet Message Access Protocol|Internet Message Access Protocol (IMAP)]] e-mail retrieval protocol, version 4 rev 1, for "international" mailbox names.{{Ref RFC|3501|section=5.1.3 "Mailbox International Naming Convention"
 |quote=In modified UTF-7, printable [[US-ASCII]] characters, except for "&", represent themselves….  The character "&" (0x26) is represented by the two-octet sequence "&-". All other characters… are represented in modified BASE64….
}} 
The following version, IMAP version 4 rev 2, uses UTF-8 instead.{{Ref RFC|9051|section=5.1. "Mailbox Naming" |quote=In IMAP4rev2, mailbox names are encoded in Net-Unicode (this differs from IMAP4rev1).}}