RFC Errata
RFC 3696, "Application Techniques for Checking and Transformation of Names", February 2004
Source of RFC: INDEPENDENT
Errata ID: 4002
Status: Held for Document Update
Type: Technical
Publication Format(s) : TEXT
Reported By: Brandon Gabbert
Date Reported: 2014-05-28
Held for Document Update by: Nevil Brownlee
Date Held: 2014-08-25
Section 3 says:
The exact rule is that any ASCII character, including control characters, may appear quoted, or in a quoted string. When quoting is needed, the backslash character is used to quote the following character. For example Abc\@def@example.com is a valid form of an email address. Blank spaces may also appear, as in Fred\ Bloggs@example.com The backslash character may also be used to quote itself, e.g., Joe.\\Blow@example.com In addition to quoting using the backslash character, conventional double-quote characters may be used to surround strings. For example "Abc@def"@example.com "Fred Bloggs"@example.com are alternate forms of the first two examples above.
It should say:
The exact rule is that any ASCII character, including control characters, may appear quoted, or in a quoted string. When quoting is needed, the backslash character is used to quote the following character. For example Abc\@def@example.com is a valid form of an email address. Blank spaces may also appear, as in Fred\ Bloggs@example.com The backslash character may also be used to quote itself, e.g., Joe.\\Blow@example.com In addition to quoting using the backslash character, conventional double-quote characters may be used to surround strings. For example "Abc@def"@example.com "Fred Bloggs"@example.com "Joe.\\Blow"@example.com are alternate forms of the examples above.
Notes:
Errata 3563 is incorrect. The first two suggested additions it makes to the spec are actually already present in the original spec just one paragraph down. The third and final suggested addition (allowing an unquoted backslash in a quoted string), while appearing to comport with this RFC, violates RFC 2822 (the reference document for this section). While the suggested email address is valid, it is not equivalent to the original.
RFC 2822 sections 3.2.1, 3.2.2, and 3.2.5 define quoted-string as consisting of any unquoted ASCII character except for backslash and double quote, and any backslash-quoted ASCII character including backslash and double quote.
Thus, while it is correct that
"Joe.\Blow"@example.com
is a valid email address, it is not equivalent to
Joe.\\Blow@example.com
as the \B in the first should be interpreted as a quoted B, not as an illegally unquoted backslash followed by a B. The quoted equivalent of
Joe.\\Blow@example.com
is
"Joe.\\Blow"@example.com
This example was probably left out of the original spec because the quoted-string version differs from the original only in the quotes themselves.