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Sendmail, 3rd Edition by Bryan Costales

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Overview

Options are declared in the configuration file by beginning a line with the letter O:

OQ/var/spool/mqueue                  single-character name (prior to V8.7)

Prior to V8.7 sendmail, option names could only be a single character. The short (single-character) name of the option (here, Q) immediately follows the O with no intervening space. The value assigned to a single-character option immediately follows the option letter with no intervening space.

Beginning with V8.7 sendmail, option names can be composed of multiple characters:

O QueueDirectory=/var/spool/mqueue   multicharacter name (beginning with V8.7)
 exactly one space

To use multicharacter names, you must separate the name (here, QueueDirectory) from the O command with exactly one space character.[2] The value assigned to the multicharacter option follows an equal sign. The equal sign can be surrounded by optional spaces.

Some options have both a single- and a multicharacter name, in which case the two names are equivalent, and the modern multicharacter name is preferred:

OQ/var/spool/mqueue                  define location of queue directory
O QueueDirectory=/var/spool/mqueue the same and preferred

The short name is retained so that old configuration files will still work with newer versions of sendmail. They should, however, be considered deprecated, and support for them might disappear in future releases of sendmail. Most options (especially the newer ones) only have multicharacter names:

O ServiceSwitchFile=/etc/service.switch    only multicharacter ...

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