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Sendmail, 3rd Edition by Bryan Costales

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?flags? in Header Definitions

The name part of the H configuration command can be prefixed with a list of flags. This list, if present, must be surrounded by ? characters:

H?flags?name:field

The ? characters must immediately follow the H and immediately precede the name with no intervening spaces. If a space precedes the first ?, that ? is misinterpreted as part of the header name, rather than as the start of a list of flags, and this error message is printed:

header syntax error, line " ?flags?name: field"
                           note leading space

If the first ? is present but the second is absent, sendmail prints the same error message and skips that H configuration command. The flags that are listed between the ? characters correspond to flags that are listed with delivery agent F= equates. When processing a mail message for forwarding or delivery, sendmail adds a header line if a flag is common to both the H definition list of flags and the delivery agent’s list of flags. For example:

H?P?Return-Path: <$g>

This H definition begins with a P flag. This tells sendmail to add this header line to the mail message only if a selected delivery agent also contains that flag. Because the Return-Path: header (Return-Path:) should be added only during final delivery, the P flag appears only in the prog and local delivery agent definitions:

Mprog,  P=/bin/sh,   F=lsDFMeuP,  S=10, R=20, A=sh -c $u
Mlocal, P=/bin/mail, F=rlsDFMmnP, S=10, R=20, A=mail -d $u
                               note

No check is made to ensure that the H flags correspond to ...

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