To provide compatibility with old configuration files, the V line has been added to define some very basic semantics of the configuration file. These are not intended to be long term supports; rather, they describe compatibility features which will probably be removed in future releases.
N.B.: these version levels have nothing to do with the version number on the files. For example, as of this writing version 8 config files (specifically, 8.7) used version level 6 configurations.
Old configuration files are defined as version level one. Version level two files make the following changes:
Version level three files allow # initiated comments on all lines. Exceptions are backslash escaped # marks and the $# syntax.
Version level four configurations are completely equivalent to level three for historical reasons.
Version level five configuration files change the default definition of $w to be just the first component of the hostname.
Version level six configuration files change many of the local processing options (such as aliasing and matching the beginning of the address for `|' characters) to be mailer flags; this allows fine-grained control over the special local processing. Level six configuration files may also use long option names. The ColonOkInAddr option (to allow colons in the local-part of addresses) defaults on for lower numbered configuration files; the configuration file requires some additional intelligence to properly handle the RFC 822 group construct.
The V line may have an optional / vendor to indicate that this configuration file uses modifications specific to a particular vendor. You may use /Berkeley to emphasize that this configuration file uses the Berkeley dialect of sendmail.