RFC 2047 defines the encoded-words mechanism for encoding non-ASCII character in MIME documents. It specifies that whitespace characters (space and tabs) are not allowed inside the encoded-word.
However, RFC 5322 for parsing email MIME documents specifies that long header lines should be "folded". Should this folding take place before or after encoded-words decoding?
I recently received an email where encoded-text part of the header had a newline in it, like this:
Header: =?UTF-8?Q?=C3=A5
=C3=A4?=
Would this be valid?
Of course emails can be invalid in lots of exciting ways and the parser needs to handle that, but it's interesting to know the "correct" way. :)
I misread the question and answered as if it was a different sort of whitespace. In this case the white space appears inside the MIME word, not multiple ones separated by white space.
This sort of thing is explicitly disallowed. From the introduction to the format in RFC2047:
2. Syntax of encoded-words
An 'encoded-word' is defined by the following ABNF grammar. The
notation of RFC 822 is used, with the exception that white space
characters MUST NOT appear between components of an 'encoded-word'.
And then later on in the same section:
IMPORTANT: 'encoded-word's are designed to be recognized as 'atom's
by an RFC 822 parser. As a consequence, unencoded white space
characters (such as SPACE and HTAB) are FORBIDDEN within an
'encoded-word'. For example, the character sequence
=?iso-8859-1?q?this is some text?=
would be parsed as four 'atom's, rather than as a single 'atom' (by
an RFC 822 parser) or 'encoded-word' (by a parser which understands
'encoded-words'). The correct way to encode the string "this is some
text" is to encode the SPACE characters as well, e.g.
=?iso-8859-1?q?this=20is=20some=20text?=
The characters which may appear in 'encoded-text' are further
restricted by the rules in section 5.
Earlier answer
This sort of thing is explicitly allowed. Headers with MIME words should be 76 characters or less and folded if needed. RFC822 folded headers are indented second and any additional lines. RFC2047 headers are supposed to only indent one space. The whitespace between ?= on the first line and =? should be suppressed from output.
See the example on the bottom of page 12 of the RFC:
encoded form displayed as
---------------------------------------------------------------------
(=?ISO-8859-1?Q?a?= (ab)
=?ISO-8859-1?Q?b?=)
Any amount of linear-space-white between 'encoded-word's,
even if it includes a CRLF followed by one or more SPACEs,
is ignored for the purposes of display.
I need to use/store a delimiter separated value string (not csv) of email addresses. I need to choose a delimiter that is safe.
E.g. bar#foo.com,baz#foo.com, - comma in this e.g. is unsafe as it's valid within an email address.
It seems that almost anything is allowed in an email address, especially now with internationalized email addresses.
What is a safe delimiter to use without jumping through hoops because of corner cases? I can't find a character in the RFC which which is expressly invalid (but there are lots of email related related RFCs, so I'm not sure which to consult).
Where/how will you be storing the string and what will the delimiter be used for?
You could use a non-visible ascii character such as the CR (Ascii 13) or Tab (Ascii 9).
I originally used \ because that is an escape character, however it is allowed if escaped. #MatWalker's answer recommends stuff like CR or LF etc, but those are allowed too, if they are escaped.
Escaping and replacing and unescaping got a bit complicated. So right now I'm using control character STX (i.e. "Start of Text", decimal 2).
Although the RFC doesn't mention (from what I've seen) whether control charactes are valid/invalid, there doesn't seem to be anything that makes it a bad choice. It does say that control chars are "discouraged", but not prohibited for header fields.
Is an email valid if it ends with a digit ? e.g.
cat#cool.thing89
With the knowledge that an email consists of [local section]#[domain section]...
An unbracketed domain consists of labels separated by periods and less than 253 characters.
Labels can be quoted or unquoted (unquoted labels must consist of a-z, A-Z, 0-9, or any of !#$%&'*+-/=?^_{|}~ or ` and have at least one character)
The right-most label must be all alphabetic.
...it is not possible for an email to end in a digit, as the domain must either be bracketed (surrounded with []) or end with an all alphabetic label or one that ends with a quote. Also, here's the relevant RFC:
The top level domain must be all alphabetic. (RFC 3696, section 2)
Taken from here.
It is not valid, until the day that a TLD ending in a digit is created.
But I'm at a loss for the thought process behind the question. Do you really need code that rejects A#B.C1 but accepts A#B.C?
Does IRC support internationalized (UTF-8) room names?
How? A pointer to documentation or a spec would be welcome.
According to RFCs 1459 and 2812, channel names can be made up of any bytes other than \x00 (NUL), \x07 (BELL), \x0A (LF), \x0D (CR), \x20 (space), , and :. How those bytes are interpreted is entirely up to the client. IRC doesn't impose any specific restrictions on the encoding.
No specific character set is specified. The protocol is based on a set of codes which are composed of eight (8) bits, making up an octet. Each message may be composed of any number of these octets; however, some octet values are used for control codes, which act as message delimiters.
You need to use IRCX protocol extensions from Microsoft
Basically you add a '%' character append your UTF-8 string and perform a post-processing on the result replacing characters using this table:
\b " " (blank)
\c ","
\\ "\"
\r CR
\n LF
\t TAB
Here is the link to the specification:
https://datatracker.ietf.org/doc/html/draft-pfenning-irc-extensions-04#page-5
How many characters are allowed to be in the subject line of Internet email?
I had a scan of The RFC for email but could not see specifically how long it was allowed to be.
I have a colleague that wants to programmatically validate for it.
If there is no formal limit, what is a good length in practice to suggest?
See RFC 2822, section 2.1.1 to start.
There are two limits that this
standard places on the number of
characters in a line. Each line of
characters MUST be no more than 998
characters, and SHOULD be no more than
78 characters, excluding the CRLF.
As the RFC states later, you can work around this limit (not that you should) by folding the subject over multiple lines.
Each header field is logically a
single line of characters comprising
the field name, the colon, and the
field body. For convenience however,
and to deal with the 998/78 character
limitations per line, the field body
portion of a header field can be split
into a multiple line representation;
this is called "folding". The general
rule is that wherever this standard
allows for folding white space (not
simply WSP characters), a CRLF may be
inserted before any WSP. For
example, the header field:
Subject: This is a test
can be represented as:
Subject: This
is a test
The recommendation for no more than 78 characters in the subject header sounds reasonable. No one wants to scroll to see the entire subject line, and something important might get cut off on the right.
RFC2322 states that the subject header "has no length restriction"
but to produce long headers but you need to split it across multiple lines, a process called "folding".
subject is defined as "unstructured" in RFC 5322
here's some quotes ([...] indicate stuff i omitted)
3.6.5. Informational Fields
The informational fields are all optional. The "Subject:" and
"Comments:" fields are unstructured fields as defined in section
2.2.1, [...]
2.2.1. Unstructured Header Field Bodies
Some field bodies in this specification are defined simply as
"unstructured" (which is specified in section 3.2.5 as any printable
US-ASCII characters plus white space characters) with no further
restrictions. These are referred to as unstructured field bodies.
Semantically, unstructured field bodies are simply to be treated as a
single line of characters with no further processing (except for
"folding" and "unfolding" as described in section 2.2.3).
2.2.3 [...] An unfolded header field has no length restriction and
therefore may be indeterminately long.
after some test: If you send an email to an outlook client, and the subject is >77 chars, and it needs to use "=?ISO" inside the subject (in my case because of accents) then OutLook will "cut" the subject in the middle of it and mesh it all that comes after, including body text, attaches, etc... all a mesh!
I have several examples like this one:
Subject: =?ISO-8859-1?Q?Actas de la obra N=BA.20100154 (Expediente N=BA.20100182) "NUEVA RED FERROVIARIA.=
TRAMO=20BEASAIN=20OESTE(Pedido=20PC10/00123-125),=20BEASAIN".?=
To:
As you see, in the subject line it cutted on char 78 with a "=" followed by 2 or 3 line feeds, then continued with the rest of the subject baddly.
It was reported to me from several customers who all where using OutLook, other email clients deal with those subjects ok.
If you have no ISO on it, it doesn't hurt, but if you add it to your subject to be nice to RFC, then you get this surprise from OutLook. Bit if you don't add the ISOs, then iPhone email will not understand it(and attach files with names using such characters will not work on iPhones).
Limits in the context of Unicode multi-byte character capabilities
While RFC5322 defines a limit of 1000 (998 + CRLF) characters, it does so in the context of headers limited to ASCII characters only.
RFC 6532 explains how to handle multi-byte Unicode characters.
Section 3.4 ( Effects on Line Length Limits ) states:
Section 2.1.1 of [RFC5322] limits lines to 998 characters and
recommends that the lines be restricted to only 78 characters. This
specification changes the former limit to 998 octets. (Note that, in
ASCII, octets and characters are effectively the same, but this is
not true in UTF-8.) The 78-character limit remains defined in terms
of characters, not octets, since it is intended to address display
width issues, not line-length issues.
So for example, because you are limited to 998 octets, you can't have 998 smiley faces in your subject line as each emoji of this type is 4 octets.
Using PHP to demonstrate:
Run php -a for an interactive terminal.
// Multi-byte string length:
var_export(mb_strlen("\u{0001F602}",'UTF-8'));
// 1
// ASCII string length:
var_export(strlen("\u{0001F602}"));
// 4
// ASCII substring of four octet character:
var_export(substr("\u{0001F602}",0,4));
// '😂'
// ASCI substring of four octet character truncated to 3 octets, mutating character:
var_export(substr("\u{0001F602}",0,3));
// 'â–’'
I don't believe that there is a formal limit here, and I'm pretty sure there isn't any hard limit specified in the RFC either, as you found.
I think that some pretty common limitations for subject lines in general (not just e-mail) are:
80 Characters
128 Characters
256 Characters
Obviously, you want to come up with something that is reasonable. If you're writing an e-mail client, you may want to go with something like 256 characters, and obviously test thoroughly against big commercial servers out there to make sure they serve your mail correctly.
Hope this helps!
What's important is which mechanism you are using the send the email. Most modern libraries (i.e. System.Net.Mail) will hide the folding from you. You just put a very long email subject line in without (CR,LF,HTAB). If you start trying to do your own folding all bets are off. It will start reporting errors. So if you are having this issue just filter out the CR,LF,HTAB and let the library do the work for you. You can usually also set the encoding text type as a separate field. No need for iso encoding in the subject line.