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. The value assigned to the multicharacter option follows an equal sign. The equal sign can be surrounded by optional spaces.
 If the short option name is a space, sendmail presumes that the option name will be a multicharacter one.
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 form available
The values for some options are strings (such as /tmp). The values for others can be numbers (such as 3), time durations (such as 3d for three days), or a boolean value (such as True). There are no hard rules for which type of value goes with which option. Instead, you'll need to look up each option in Table 24-4, in Section 24.5, and use the type indicated there.