All headers, unless otherwise noted, can have their functionality extended by the use of additional, optional subfields.
Systems parsing a header MUST parse out any subfields on the header, and either interpret them, ignore them or cause an error as described below.
Subfields may be appended to the end of any header by the use of a semicolon (;) and a series of "attribute = value" parameters following the syntax of MIME headers and "parameters" defined in RFC2045, except as defined below:
If the attribute begins with "M-" then interpretation of it is non-optional. Systems that do not understand the attribute should ignore the entire header and MAY give a warning to the user or abort processing of the article.
If the attribute begins with "MM-" then interpretation of it is vital to processing of the entire article. Systems that do not understand such an attribute (and are attempting to process the header) SHOULD either abort processing of the article, or provide a warning to users that the article is probably not being handled properly.
(Note that a header meant only for newsreaders, and not processed by news transports, can thus contain an "MM-" attribute that does not cause the transport to abort handling the article.)
Programs processing a subfield using "=" MUST ignore the subfield if they do not understand the attribute word.
It should be noted that values may contain quoted strings and comments. It is not sufficient to simply parse a line using semicolons. The primary field of a given header entry may also use such quoting conventions.
The following "plain text" headers MUST not be extended with optional subfields. These fields MAY contain semicolons, but they are part of the ordinary text.
Subject, Summary, Organization
In some cases, existing software is unable to understand extension fields. As such, the following fields MUST NOT be extended prior to Extended-Header-Date, though all software written to this standard MUST deal with extensions:
From, Path, Date, Newsgroups, Distribution, Message-ID, References, Control, Expires, Reply-to, Sender, Followup-to, Supersedes, Xref
The following headers may be used in extended form immediately, though no specific extensions to their standard definition are currently defined.
All MIME headers which define the use of subfields.
Lines
Approved
Keywords
All other new headers defined in this standard which were not present in RFC1036. (List here...)