Header Field Contents
The field
of the H
configuration command
can contain any ASCII characters, including whitespace and
newlines that result from joining.[436] For most headers, however, those characters must
obey the following rules for grouping:[437]
- Atom
In the header
field
, space characters separate one item from another. Each space-delimited item is further subdivided by specials (described next), into atoms:smtp ← an atom foo@host ← atom special atom Babe Ruth ← atom atom
An atom is the smallest unit in a header and cannot contain any control characters. When the
field
is an address, an atom is the same thing as a token (see Chapter 18 on page 648).- Specials
The special characters are those used to separate one component of an address from another. They are internally defined as:
( ) < > @ , ; : \ " . [ ]
A special character can be made nonspecial by preceding it with a backslash character. For example:
foo;fum ← atom special atom foo\;fum ← one atom
The space and tab characters (also called linear-whitespace characters) are also used to separate atoms and can be thought of as specials.
- Quoted text
Quotation marks can be used to force multiple items to be treated as a single atom. For example:
Babe Ruth ← atom atom "Babe Ruth" ← a single atom
Quoted text can contain any characters except for the quotation mark (
"
) and the backslash character (\
).- Any text
Some headers, such as
Subject:
(Subject: on page 1166), impose minimal rules on the text in the headerfield
. For such headers, atoms, ...
Get sendmail, 4th Edition now with the O’Reilly learning platform.
O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.