The field of the H configuration command can contain any ASCII characters, including whitespace and newlines that result from joining. For most headers, however, those characters must obey the following rules for grouping:
 Beginning with V8.10, the field can also contain a call to a rule set for special processing (Section 25.5).
 This discussion is adapted from RFC2822.
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).
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.
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 (\).
Some headers, such as Subject: (Subject:), impose minimal rules on the text in the header field. For such headers, atoms, specials, and quotes have no significance, and the entire field is taken as arbitrary text.
The detailed requirements of each header name are covered at the end of this chapter.
Macros can appear in any position in the field of a header definition line. Such macros are not expanded (their values tested or used) until mail is queued or delivered. For the meaning of each macro name and a description of when each is given a value, see Chapter 21.
Only two macro prefixes can be used in the field of header definitions:
The $ prefix tells sendmail to replace the macro's name with its value at that place in the field definition.
The $? prefix tells sendmail to perform conditional replacement of a macro's value.
For example, the following header definition uses the $ prefix to insert the value of the macro x into the header field:
The macro $x ($x) contains as its value the full name of the sender.
When the possibility exists that a macro will not have a value at the time the header line is processed, the $? conditional prefix (Section 21.6) can be used:
HReceived: $?sfrom $s $.by $j ($v/$V)
Here, the $? prefix and $. operator cause the text:
to be inserted into the header field only if the macro s has a value. $s can contain as its value the name of the sending site.
Recall that the backslash escape character (\) is used to deprive the special characters of their special meaning. In the field of header definitions the escape character can be used only inside quoted strings (see next item), in domain literals (addresses enclosed in square bracket pairs), or in comments (discussed later). Specifically, this means that the escape character cannot be used within atoms. Therefore, the following is not legal:
Full\ Name@domain not legal
Instead, the atom to the left of the @ must be isolated with quotation marks:
"Full Name"@domain legal
Recall that quotation marks (") force arbitrary text to be viewed as a single atom. Arbitrary text is everything (including joined lines) that begins with the first quotation mark and ends with the final quotation mark. The following example illustrates two quoted strings:
"Full Name" "One long string carried over two lines by indenting the second" whitespace
The quotation mark character can appear inside a quoted string only if it is escaped by using a backslash:
 Note that the backslash itself cannot appear within full quotation marks.
"George Herman \"Babe\" Ruth"
Internally, sendmail does not check for balanced quotation marks. If it finds the first but not the second, it takes everything up to the end of the line as the quoted string.
When quotation marks are used in an H configuration command, they must be balanced. Although sendmail remains silent, unbalanced quotation marks can cause serious problems when they are propagated to other programs.
Comments consist of text inside a header field that is intended to give users additional information. Comments are saved internally by sendmail when processing headers, then are restored, but otherwise are not used. Beginning with V8.7 sendmail, the F=c delivery agent flag (F=c) can be used to prevent restoration of the saved comments.
A comment begins with a left parenthesis and ends with a right parenthesis. Comments can nest. The following lines illustrate a nonnested comment and a comment nested inside another:
(this is a comment) (text(this is a comment nested inside another)text)
Comments can be split over multiple lines by indenting:
(this is a comment split into two lines) whitespace
A comment (even if nested) separates one atom from another just like a space or a tab does. Therefore, the following produces two atoms rather than one:
However, comments inside quoted strings are not special, so the following produces a single atom:
Parentheses can exist inside of comments only if they are escaped with a backslash:
<email@example.com> (The happy administrator ;-\)) note
Many of the special characters that are used in the header field and in addresses need to appear in balanced pairs. Table 25-2 shows these characters and the characters needed to balance them. Failure to maintain balance can lead to failed mail. Note that only parentheses can be nested. None of the other balanced pairs can nest.
You have already seen the quoted string and comments. The angle brackets (< and >) are used to specify a machine-readable address, such as <firstname.lastname@example.org>. The square brackets ([ and ]) are used to specify a direct Internet address (one that bypasses normal DNS name lookups) such as [18.104.22.168].
The sendmail program gives warnings about unbalanced characters only when it is attempting to extract an address from a header definition, from the header line of a mail message, or from the envelope. Beginning with V8.6, when sendmail finds an unbalanced condition, it tries to balance the offending characters as rationally as possible. Regardless of whether it can balance them, it prints one of the following warning messages:
Unbalanced ')' Unbalanced '>' Unbalanced '(' Unbalanced '<' Unbalanced '"'
If it did not succeed in balancing them, the mail will probably bounce.