<p>长头字段的处理在<a href="https://tools.ietf.org/html/rfc2822#section-2.2.3" rel="nofollow noreferrer">section 2.2.3 of RFC 2822 "Internet Message Format"</a>中定义。该部分在过时的<a href="https://tools.ietf.org/html/rfc5322#section-2.2.3" rel="nofollow noreferrer">RFC 5322</a>中保持不变。在</p>
<blockquote>
<p>2.2.3. Long Header Fields</p>
<p>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:</p>
<pre class="lang-none prettyprint-override"><code>Subject: This is a test
</code></pre>
<p>can be represented as:</p>
<pre class="lang-none prettyprint-override"><code>Subject: This
is a test
</code></pre>
<p>Note: Though structured field bodies are defined in such a way that
folding can take place between many of the lexical tokens (and even
within some of the lexical tokens), folding SHOULD be limited to
placing the CRLF at higher-level syntactic breaks. For instance, if a
field body is defined as comma-separated values, it is recommended
that folding occur after the comma separating the structured items in
preference to other places where the field could be folded, even if it
is allowed elsewhere.</p>
<p><strong>The process of moving from this folded multiple-line representation of
a header field to its single line representation is called
"unfolding". Unfolding is accomplished by simply removing any CRLF
that is immediately followed by WSP. Each header field should be
treated in its unfolded form for further syntactic and semantic
evaluation.</strong></p>
</blockquote>