Comments provide you with the documentation necessary to maintain the configuration file. Because comments slow down sendmail by only a negligible amount, and only at startup, it is better to overcomment than to undercomment.
lines and lines that begin with a
# character are
considered comments and are ignored. A blank line is one that
contains no characters at all (except for its terminating newline).
Indentation characters (spaces and tabs) are invisible and can turn
an apparently blank line into an empty-looking
line, which is not ignored:
# text ← a comment tabtext← a continuation line ← a blank line tab← an "empty-looking line"
Except for two special cases, pre-V8 comments occupy the entire line.
The two special cases are the
S configuration commands. The
command is composed of three tab-separated fields, the third field
being a comment that does not require a leading
Rlhs rhs comment
S command looks only for a number
following it and ignores everything else, so it can also be followed
by a comment:
S3 this is a comment
Prior to V8, no other commands allow comments to follow on the same line:
CWlocalhost mailhost # This won't work prior to V8